Granny Flat Addition Costs NSW 2026 - Get Free Quotes

Granny Flat Additions in NSW

Converting a garage into a secondary dwelling costs $60,000 to $120,000 across New South Wales, an under house or basement conversion runs $80,000 to $180,000, and an attached addition sharing a wall with the existing house costs $130,000 to $230,000. Fire and acoustic separation adds a further $6,000 to $18,000 wherever a wall ends up shared between the two dwellings, works to raise a ceiling or lower a floor add $8,000 to $40,000 on top of that, and structural work to the existing house adds $8,000 to $45,000 again where it proves necessary once the building has been opened up. Approvals and certification add a further $4,000 to $9,000 on top of every figure above, regardless of which of the three routes you eventually take, as set out at granny flat rules. None of these figures makes any allowance for furniture, landscaping, or finding somewhere else on the property to park the car once the garage has gone.

Households choose these routes expecting to undercut a detached build, and the arithmetic does not always cooperate. A detached dwelling costs $140,000 to $250,000 to build from scratch, so a garage conversion sitting at the bottom of its range genuinely does save money, while an under house conversion landing at the top of its range saves nothing whatsoever. Which end you land on is determined by conditions that exist before you start rather than by decisions you make afterwards. The era in which the garage was originally built will tell you considerably more about the likely cost than its floor area ever will. Concrete and brick garages put up during the 1960s and 1970s were commonly built to a lower ceiling than anything constructed in the decades since. Get one quote for each viable route before settling on any of them.

The single most useful thing you can do is have the ceiling height and the floor level measured by somebody who knows what they are looking for, before you commission drawings or set a budget. Those two dimensions between them decide whether a conversion turns out to be a straightforward project or an expensive rebuild in disguise, and neither one can be changed cheaply once the building exists. Building new instead is priced at granny flat costs. A certifier or a sufficiently experienced builder will complete both measurements in a single visit and tell you honestly whether the project is viable at all. Paying somebody properly for that one visit is the best value available anywhere in the entire exercise.

The Three Routes

A garage conversion turns an existing detached or attached garage into a dwelling, and it is the route households consider first because the structure looks complete. Walls, a roof and a slab all exist already, side access has usually been established for years, and the position of the building within the block was settled when the house went up. The problems are that garages are built to a lower standard than dwellings in every respect that matters, and the extent of that gap is not visible from inside the building. A detached garage sitting in the rear corner of a block is a very much better candidate than one built underneath the main roof of the house. The more genuinely separate the structure already happens to be, the less separation work you will end up having to do to it later. Ask when the garage was built, since the era tells you more about the likely ceiling height than any photograph will.

An under house or basement conversion uses space beneath an existing dwelling, which is common on sloping sites through the northern beaches, the Illawarra and the Blue Mountains. It preserves the yard entirely, which is its single great advantage over every other option, and it brings with it the hardest set of technical problems of any route available. Head height, damp, natural light, ventilation and access all need solving simultaneously, and a failure on any one of them stops the project. Access is frequently the forgotten problem in these projects, since a staircase leading down from inside the house does not give a separate dwelling anything resembling its own entry. Forming an external door and a usable path leading to it on a steeply sloping site can easily mean retaining walls, steps and a good deal of excavation. Access is the forgotten problem, since a stair down from inside the house gives a separate dwelling no entry of its own.

An attached addition builds new space sharing a wall with the existing house, which is the most expensive route and the most predictable. Because the structure is new throughout it can be built correctly from the outset rather than corrected afterwards at expense, and the ceiling height, the insulation, the damp proofing and every window position are decided by you. What you take on instead is the junction between new and existing, which is where attached projects find their difficulties. It is also the route most likely to require full council consent rather than the faster certifier pathway, depending entirely on how the new work sits on the block. Setbacks measured from the face of the new wall rather than from the old one catch households out with some regularity.

Choosing between the three is less a preference than a reading of your site. A household with a high ceilinged brick garage sitting on a level block with good separation from the house has an excellent conversion candidate on their hands, while one with a low garage of tilt slab construction, where the concrete wall panels were cast flat and lifted upright, falling away toward the street does not. Have somebody assess the existing structure before you develop any attachment to a particular route. Ask a builder which route they would take on your particular block before you have decided yourself, and check credentials the way granny flat builders describes. Their answer, and rather more importantly how quickly they arrive at it, tells you a great deal about how often they have done this kind of work before.

All three still require approval and none of them proceeds because the structure already exists. The proposal goes to a certifier, mandatory inspections fall at set points during the work, and nobody may move in until an occupation certificate is issued, just as with a new building. The pathway options are set out at granny flat approval. Work already carried out on the property without approval complicates matters considerably and generally has to be resolved before anything new is allowed to proceed. Building information certificates exist for precisely that situation, and obtaining one takes time you will need to allow for in the programme. Work already done without approval has to be resolved first, and a building information certificate takes time to obtain.

Becoming Habitable Space

The governing idea behind every conversion is that a garage is classified as non habitable space and a dwelling is not. Non habitable space carries no requirement to keep anybody warm, dry, ventilated or comfortable, and it gets built accordingly, with uninsulated walls, minimal weather sealing, no damp proofing beneath the slab and frequently no windows at all. Converting it means bringing every one of those elements up to a standard the original construction was never intended to reach. That single distinction between habitable and non habitable space explains very nearly every cost that surprises households on a conversion project. Once you hold that idea firmly in mind, the quotes you receive stop looking unreasonable and start looking entirely explicable.

Natural light and ventilation are required in habitable rooms and most garages have neither. Forming window openings in masonry walls means lintels, structural certification and a good deal of making good afterwards, and doing the same thing in a tilt slab or precast concrete wall is a considerably larger exercise again. Where the garage happens to be attached on two or even three of its sides, there may be very few walls available to put any windows into at all, which constrains the layout before it is drawn. That constrains the internal layout before anybody has drawn it, which is why the assessment comes before the work covered at granny flat floor plans.

Thermal performance has to be demonstrated and a converted garage starts a long way behind. Uninsulated masonry walls, an uninsulated roof space with no Bradford or Earthwool batts, a slab with no edge insulation and single glazed openings all count against the assessment together, and the remedies involve furring out walls, losing internal floor area in the process, and specifying considerably better glazing than you expected to buy. This work is invisible in the finished room and appears in full on the quote. Furring out a wall so that it can take insulation costs you somewhere between seventy and ninety millimetres of internal width on every single wall treated. Across a single garage that can quietly remove a square metre of floor area from a building which had none at all to spare in the first place.

The sustainability assessment applies to conversions in the same way it applies to new work. It considers the building fabric, the glazing, the orientation and the fixed appliances together, and a garage facing the wrong direction with very limited window options can be genuinely difficult to bring through it. Commission the assessment early rather than treating it as paperwork at the end, and the process is described at granny flat complying development. A garage whose only available wall faces south is the genuinely difficult case, since the glazing that would help the assessment most cannot be placed anywhere useful. Discuss it with the assessor before the layout is fixed.

Everything above applies equally to under house conversions and rather more severely. Below ground walls bring damp with them, along with limited window opportunity, restricted ventilation and frequently no possibility of achieving a compliant window in a bedroom at all. Establish what can be achieved in that space before assuming a bedroom is possible there. Mechanical ventilation can solve a portion of the problem and it cannot solve the whole of it. A room that relies entirely on a fan running in order to remain habitable is a room somebody will find distinctly unpleasant by year three.

Ceiling Height

Ceiling height ends more conversion projects than every other factor combined, and it is measured in five minutes. Habitable rooms are generally required to achieve somewhere around 2.4 metres, with lower figures permitted in certain circumstances and for particular room types, and your certifier will confirm exactly what figure applies to your own situation. Older structures and anything under a low pitched roof are the usual offenders. Measure it before anything else at all, because every later decision including those at granny flat approval depends on the answer. Take the measurement yourself with a tape before paying anybody for an opinion.

Measure from the finished floor to the underside of the ceiling rather than to the roof structure, and measure in several places. A garage with an exposed truss or an open rafter overhead may look perfectly generous while the actual clear height beneath the bottom chord, the horizontal timber running along the base of each truss, falls well under what a habitable room requires. Where a ceiling will be installed as part of the conversion, the measurement that matters is what remains once it is in place. Remember that any new floor build up will eat directly into whatever height you measure today. Where a screed or a raised timber floor is going in, subtract that full thickness before concluding the height works.

Gaining height by raising the roof is expensive and it is a real structural project rather than an adjustment. New wall plates, new roof framing, new tie downs, revised bracing and fresh engineering certification are all involved in the exercise, and $18,000 to $40,000 is a realistic range depending on the roof form and the size of the building. Where the garage is attached to the house, the junction with the existing roof adds further complexity. Expect the roofing, the guttering and quite possibly the stormwater arrangements to be affected as well.

Gaining height by lowering the floor is usually worse than raising the roof. Excavating below an existing slab means undermining the very footings that support the walls standing above it, which requires underpinning, engineering input and a great deal of care, and on a building this small it frequently costs more than demolishing and rebuilding. Treat this option with real scepticism when somebody proposes it casually. Ask specifically whether underpinning has been included in the figure quoted, because that single answer separates a real quote from an optimistic one.

Under house spaces have the same problem in a more acute form. A subfloor offering 2.1 metres of head height cannot become a bedroom without excavation of some kind, and excavating beneath a house you fully intend to keep living in throughout is substantial work. Get the head height measured before anything else happens, because a negative result here saves you the entire cost of investigating everything downstream. Removing soil from under an occupied dwelling also raises questions about the existing footings, which an engineer rather than a builder should answer.

Damp and Floor Levels

Garage slabs are commonly poured without a damp proof membrane beneath them, because a garage does not need one. A dwelling most certainly does, since moisture rising through a slab destroys floor coverings, produces persistent damp and creates the conditions for mould, and a slab already sitting in the ground cannot have a membrane retrofitted beneath it afterwards. The remedy is a topical membrane or a new insulated slab laid over the existing, which costs $12,000 to $25,000 and consumes ceiling height you may not have. This is the second and equally good reason that the height measurement comes before everything else.

Garage floors are also laid to fall toward the door so that water drains out, and that fall is typically fifty to a hundred millimetres across the length of the slab. A dwelling needs a level floor throughout, and levelling one means either a screed that adds thickness and reduces the head height further, or a raised timber floor that achieves the same thing rather more expensively. Neither option is available where the ceiling is already marginal. The two problems interact directly with each other, which is exactly what makes a low garage with a falling slab such a poor candidate.

The relationship between the floor level and the ground outside matters more than households expect. A dwelling floor generally needs to sit above the adjacent ground level in order to keep surface water out of the building, and a garage slab is frequently level with the driveway alongside it or slightly below it. Where that is the case, drainage works, a threshold detail or raising the floor all become necessary, and each of them costs money and head height. Every one of those costs money, and most of them cost you head height into the bargain. Look at where water sits in the driveway after heavy rain.

Under house conversions face damp from the walls as well as the floor. Below ground masonry sitting in direct contact with soil transmits moisture continuously and indefinitely, and tanking, which is the application of a waterproof barrier to a below ground wall, is specialist work that has to be carried out from the soil side rather than the room side to be effective at all. Doing it badly produces a room that smells and a problem that recurs indefinitely, and remedial options sit at rising damp assessment and repairs. External tanking means excavating along the side of the house itself, which is why internal systems get proposed so often and why they so frequently disappoint.

Have somebody assess the slab and the surrounding drainage before committing to any conversion. A moisture reading, a look at where the stormwater currently goes, and an honest opinion about the fall running across the floor together take about an hour and are the cheapest money you will spend on the entire project. Drainage works are covered at stormwater drainage. Ask specifically where the stormwater coming off the main roof currently discharges to. A downpipe emptying directly beside the garage wall explains a great deal about a persistently damp slab, and it is cheap to redirect. Ask where the stormwater from the main roof currently discharges, since a downpipe beside the garage wall explains a great deal.

Fire and Acoustic Separation

A wall shared between a secondary dwelling and the main house has to perform in ways an internal wall does not. Fire separation between separate occupancies is required so that a fire starting in one dwelling does not immediately reach the other, and the wall itself has to achieve a rated performance verified against the relevant construction requirements. This is a construction specification rather than a lining choice, and it cannot be satisfied by adding a sheet of plasterboard. Ask which system is being proposed and ask to see the supporting documentation for it, since a certifier will check exactly that during the inspections described at granny flat complying development.

Achieving a rated wall in existing construction is more involved than building one new. The rating applies to the whole assembly including every junction at the floor, the ceiling and the roof space above, and a wall that simply stops at the ceiling line while the roof space continues uninterrupted over both dwellings achieves nothing at all. Continuing the separation into the roof space is frequently the most awkward part of an attached project. Where the two buildings share one continuous roof cavity between them, that work has to happen in a very confined space indeed.

Penetrations through a rated wall all need treating. Power points, switches, pipes, ducts and any opening whatsoever reduce the performance of the assembly unless each one is detailed correctly, and a certifier examines them at inspection. This is a common source of rework on conversions where an electrician has run cabling through the separating wall without anybody raising the question first. Tell your electrician clearly which wall is the rated one before they begin any work.

Acoustic separation is a different requirement from fire separation and it is what determines whether the arrangement is tolerable. Airborne noise travelling through a shared wall and impact noise coming through a shared floor are both addressed separately, and the constructions that achieve them involve mass, isolation and careful sealing rather than insulation alone. Two households living either side of a poorly separated wall is a situation nobody enjoys. It also cannot be improved afterwards without opening the whole wall up again, which is precisely why it belongs in the specification from the outset.

Budget $6,000 to $18,000 for separation on any attached arrangement and treat it as unavoidable. Households frequently discover this cost late, having assumed the existing wall between the house and the garage was sufficient because it already looked like a wall. Specification of the assemblies follows published standards available through Standards Australia. A wall that has stood perfectly well for forty years is not thereby a fire rated wall.

Structure of the Existing House

Attaching new work to an existing house means engaging with a structure nobody has examined in decades. Existing footings may be shallower than current practice would allow, the wall construction may not be what the plans suggest at all, and previous alterations may well have been carried out without approval or any engineering input. All of that becomes relevant the moment you connect to it, and none of it is visible until work begins. That is the fundamental structural difference between an addition and a new building going up on clear ground.

Differential movement between old and new is the technical problem at the heart of attached additions. A new slab sitting on new footings settles quite differently from a house that finished settling decades ago, and the junction between the two has to accommodate that movement rather than attempt to resist it. Articulation joints, separate footing systems and detailing at the connection are how this is managed, and skipping it produces cracking within a year or two. Skipping any of it produces cracking within a year or two that nobody afterwards can confidently attribute to anyone. Ask to see the articulation joints marked on the drawing rather than left to the bricklayer.

Removing or altering any part of the existing structure requires engineering. Cutting an opening in a load bearing wall, removing a section of the roof structure, or altering a bracing wall all change how loads travel down to the ground, and an engineer determines what is needed in each case. Builders who propose these changes without engineering input are proposing something you should decline. Ask for the engineer's details and confirm the work has genuinely been designed rather than simply estimated on site.

Existing conditions discovered mid project are the most common source of cost overruns on additions. Termite damage in a wall frame, asbestos in old sheeting, footings sitting on uncompacted fill, or a roof structure that somebody modified previously all surface only once things have been opened up. Building a contingency of fifteen to twenty per cent into an addition budget is more important than on any new build. Agree in advance exactly how variations will be priced, since that conversation becomes very much harder once a wall has been opened up.

A pre commencement inspection of the existing structure is worth commissioning. Somebody who properly assesses the footings, the frame, the roof structure and any prior alterations gives you a far better basis for setting a budget than a builder working entirely from assumptions. Inspections of this kind sit at building inspection. It also gives you a document to hand to every builder so that all of them are pricing the same building. That one step alone usually improves the quality of the quotes you get back. It also gives every builder the same document to price against, which tightens the quotes you get back.

Entry, Metering and Outdoor Space

An attached or converted dwelling still needs its own entry, and it usually needs to be reachable without passing through the main house. That path has to remain usable at night, in heavy rain and by somebody carrying shopping in both hands, which in practice means lighting, a hard surface underfoot and adequate width. Where a converted garage opens only onto the driveway, forming an entry that reads as a front door rather than a side access is worth some thought. It affects how the occupant feels about living there more than the floor area does, and entry treatment is examined at granny flat designs. An entry that reads as a front door rather than a side access changes how the occupant feels about living there.

Private open space, meaning a usable outdoor area belonging to that dwelling alone rather than shared with the main house, is generally required and easy to overlook on a conversion. A converted garage sitting hard against a boundary with a driveway directly in front of it may have nowhere obvious to provide any, and carving an area out of the main house yard reduces what the primary dwelling gets to keep. Establish this early, since it can determine whether the proposal works at all, and area limits are covered at 60 sqm granny flats. It is also the single requirement most often overlooked on a conversion, because the building already exists and so the yard question simply never gets asked.

Separate metering is worth deciding before the electrical work is done rather than afterwards. A separately metered dwelling allows the occupant to hold their own account and pay for their own usage directly, while a shared connection means the main household pays the bill and recovers the money informally afterwards. Retrofitting separate metering after the walls are closed is considerably more expensive than allowing for it during the rough in. Decide the metering question at the same time you decide whether the dwelling will ever be tenanted.

Water and sewer connections for an attached dwelling are usually simpler than for a detached one, since the existing services are close. That is a genuine saving relative to a detached build, and it is one of the few areas where the conversion arithmetic works in your favour. The water authority may still require notification depending on the arrangement, so confirm rather than assume. Sydney Water charges for any new connection and the fee scales with the work involved, so obtain that figure early rather than treating it as incidental.

Parking, waste storage and clotheslines all still need positions on the site. A garage conversion in particular removes the parking it previously provided, and where the council expects parking to be retained this becomes a live issue rather than a detail. Ask the question before the roller door comes out, and general planning services are accessible through Service NSW. A garage conversion in particular removes the off street parking that the site previously provided. Ask the council about parking before the roller door comes out rather than after.

What Drives the Price

The condition of the existing structure determines the cost more than the size of the space does. A sound brick garage with adequate ceiling height, a level slab and two external walls available for windows converts cheaply, while a low tilt slab garage with a falling floor and one usable wall does not. Two identical footprints can differ by seventy thousand dollars for reasons entirely established before you arrived. This is why quotes on conversions vary far more than quotes for the work described at granny flats in NSW.

Whether ceiling or floor works are needed is the largest single variable. A conversion requiring neither sits at the bottom of its range, and one requiring a raised roof or a new slab moves toward the top or beyond it. Establish this before setting a budget rather than after, since it is the item most likely to change the project fundamentally. A household that discovers this late has usually already spent good money on drawings that may not be usable.

Separation requirements scale with how much wall is shared. A detached garage in the corner of the yard may need no separation at all, while a garage integrated under the main roof needs a rated assembly continuing into the roof space. The more the two buildings are entangled, the more this line grows. A freestanding garage sitting three metres clear of the house is a materially different proposition from one built under the same roof.

Services position affects the plumbing cost substantially. Where the existing sewer line runs close to the converted space the connection is straightforward, and where the bathroom ends up on the far side of the building the drainage has to travel with adequate fall. Cutting a trench through an existing slab to achieve that fall is disruptive and expensive. Position the bathroom close to the existing sewer line wherever the layout will allow it.

The unknowns are what separate an addition budget from a new build budget. A new dwelling on clear ground has few surprises once the soil report is in, while an addition has a house full of them, and the contingency should reflect that difference honestly. Anything you pay a trade to do carries consumer guarantees, which the ACCC oversees. Your contingency should reflect that difference honestly rather than optimistically. Agree how variations will be priced before a wall is opened rather than after.

Addition Costs Across Sydney

Across Sydney conversions are attractive because yard space is scarce and the existing footprint is already approved. The difference is usually decided by the era the garage was built in rather than the suburb it sits in, and the same holds for the budgets at granny flat costs. Check whether the garage was built under the original approval or added later, since an undocumented structure has to be resolved before it can become anything. A structure that appears on no approval record may need retrospective consent before it can be converted into anything habitable.

Through the western suburbs a detached garage in the rear corner of the block is the best conversion candidate available. It is separate from the house so separation requirements are minimal, it usually has three or four external walls for windows, and the ceiling height in a freestanding garage is often adequate. These convert at genuine savings against a new build. Ceiling height in a freestanding garage is more often adequate than in an integrated one.

Across the northern beaches and the upper north shore, under house space on sloping blocks is the common proposition and the hardest one. Damp, head height and getting a compliant window into a partly buried room all bite at the same time, and the projects that do succeed there are usually the ones where a builder has said no to something early. Existing subfloor ventilation is worth inspecting before anything else, because closing a well ventilated space in without addressing it creates a damp problem that was not there before. Ask what the ground does in heavy rain.

Addition Costs in Regional NSW

Regional blocks generally have room for a detached dwelling, which changes the calculation entirely. Where a new building can go up in a clear part of the yard for a comparable figure, the argument for converting an existing structure has to rest on something other than saving money. Keeping the yard intact or reusing a structure you value are perfectly good reasons, and factory built alternatives sit at prefab granny flat costs. Get one quote for each approach before deciding, since the comparison is rarely as obvious as it looks from the outside.

Converting a rural shed is a proposition worth examining carefully rather than dismissing or embracing. A shed has a slab poured for machinery rather than habitation, walls with no insulation and often no lining, a roof with no sarking, and a structural system designed for a different loading. The result can be excellent and it is a substantial construction project rather than a fit out. Price it as construction and it will make sense, and note that a supplied structure carries its own issues at granny flat kit homes.

Onsite wastewater applies to unsewered properties and a conversion adds load exactly as a new dwelling does. Existing drainage often has no capacity for a second household, and bringing it up to standard is significant work priced quite separately from everything else. Have capacity assessed before the project is designed rather than after the drawings are paid for. The assessment considers the number of bedrooms rather than the floor area, so a second bedroom can change the answer even where the building does not grow.

Regional building surveyors and certifiers are fewer and busier, which matters more on a conversion than on a new build. Conversions generate more questions requiring a determination, and each one waits on somebody who may be servicing a wide area. Build that into the programme rather than being surprised by it. Conversions generate more questions requiring a determination than new builds do.

What to Look Out For

Have the ceiling height measured before anything else. Habitable rooms generally need around 2.4 metres with some exceptions your certifier will confirm, and many garages were built below that, particularly older ones under a low pitched roof. Raising a roof costs $18,000 to $40,000 and lowering a floor is usually worse, so a negative result here saves you the cost of investigating everything downstream. Take the measurement yourself with a tape before paying anybody, since it costs nothing and tells you immediately whether the conversation is worth having.

Check the slab for damp proofing and for fall. Garage slabs are commonly poured without a membrane because a garage does not need one, and they are laid to fall fifty to a hundred millimetres toward the door so water drains out. A dwelling needs a level, dry floor, and achieving both over an existing slab costs $12,000 to $25,000 while consuming head height you may not have. It also consumes head height, which is why this check and the height check belong together.

Do not assume the existing wall between house and garage is adequate separation. Fire separation between occupancies is a rated construction specification rather than a lining choice, it has to continue into the roof space to mean anything, and every penetration through it needs detailing. Budget $6,000 to $18,000 wherever a wall is shared and treat it as unavoidable rather than negotiable. Ask where the separation terminates, since one stopping at the ceiling does nothing, and check it against the staged inspections at granny flat construction.

Keep the classification distinction in mind throughout, because it drives almost every cost above. Natural light, ventilation, insulation, weather sealing and thermal performance all have to be brought up to a standard the original construction was never intended to reach, and forming windows in masonry means lintels and structural certification. None of it is visible in the finished room and all of it appears on the quote. Furring out walls for insulation quietly costs internal width you may not have.

Carry a larger contingency than you would on a new build. Termite damage, asbestos, footings on fill and undocumented prior alterations all surface once things are opened up, and fifteen to twenty per cent is realistic. Disputes with a builder over work of this kind are heard by NCAT. Fifteen to twenty per cent is realistic on a conversion rather than pessimistic. Agree how variations will be priced before work starts.

What to Look for in a Quote in NSW

An addition quote should state what has been assumed about the existing structure. It should confirm whether the ceiling height has been measured and found adequate, say whether the slab is assumed to be sound and damp proofed, identify the separation construction proposed and where it terminates, list what structural work to the existing house is allowed for, state what happens if conditions differ once work begins, and confirm who commissions the engineering. The assumptions about the existing building are what separate a reliable quote from an optimistic one. Ask for them in writing rather than accepting a verbal reassurance, and licensing checks run through NSW Fair Trading.

Send each builder the same information about the existing structure. Provide the age of the house, any plans you hold, the measured ceiling height, what you know about the slab, whether previous alterations were approved, and photographs of the space from several angles. Builders pricing a conversion are pricing risk as much as work, and better information produces a tighter and more honest number. A vague enquiry produces a figure padded against everything they cannot see.

A builder who inspects the space before quoting is worth more than one who prices from photographs. Someone who measures the ceiling, looks at where the stormwater goes, checks the fall on the slab and asks what is behind the plasterboard is pricing what is actually there. A number produced without a site visit rests on assumptions you will fund later. Ask how long they spent on site, since twenty minutes with a tape measure is worth more than any amount of email.

Settle the ceiling height, the slab condition and the separation before you commission drawings. Have the height measured, get an opinion on damp and fall, establish where separation is required and how far it must continue, and price the structural unknowns with a proper contingency. Compare the whole exercise against building new at granny flat construction, and builders working nationally are listed on the granny flat builders hub. Price the structural unknowns with a proper contingency rather than a hopeful one.