Garage Conversion Cost 2026: Class Change, Council, and Build
Summary
The short version. What a garage conversion costs depends first on what the garage becomes. An extra room of the house, a studio with a bathroom or kitchenette that stays part of the house, and a self-contained granny flat are three different jobs with three different approval paths. For a 36m² double garage converted to a self-contained granny flat, our line-item table sums to about $58,200 to $157,900 including contingency, which is roughly $1,600 to $4,400 per square metre. The spread is not about tile choices. It is about whether the existing slab, ceiling height, and floor level can legally carry a habitable classification, because a garage is built as Class 10a and a habitable room is Class 1a, and the National Construction Code treats those as different buildings. Every conversion to habitable use needs building approval. Planning approval depends on the end use and the state, and it is most likely for a granny flat. Budget six to nine months for a granny flat that goes through planning, less for a room that only needs building approval, and do the class-change feasibility before you spend a dollar on anything else.
Table of Contents
Every dollar in this article assumes approved Class 1a work with an occupancy certificate. It is not the cost of lining a garage and sleeping in it. Cheap quotes skip the class change. These numbers do not.
A client in Western Sydney rang us about converting a double garage under the main roof. He had a quote from a renovation outfit for $38,000, all inclusive, four weeks. Nice flooring in the render. What the quote did not mention was that the garage slab sat 40mm below the external paving, had no vapour barrier under it, and the ceiling measured 2.28 metres, below the 2.4 metre minimum for a habitable room. Three problems, each one fatal to a habitable classification without additional work. By the time we had drawn a compliant solution, the real number was closer to $86,000.
That gap is the story of garage conversions in Australia. The room already exists, so the cost feels like it should be cosmetic. It is not. A garage is a Class 10a non-habitable structure. A bedroom, a studio, or a secondary dwelling is Class 1a. Changing between those two classifications under the National Construction Code brings in waterproofing, thermal performance, light, ventilation, and structural requirements that were never designed into the original garage, because they did not have to be.
Where you land on cost is decided almost entirely before a trade shows up, and the first decision is what the garage is going to be.
What a garage conversion actually costs in 2026
We track quotes across NSW, VIC, QLD, and TAS. The first thing that splits them is the end use, so this article keeps three jobs separate from here to the FAQs.
A. Extra habitable room of the existing house
A bedroom, study, or office with no wet area. The garage becomes part of the house you already live in. There is no second dwelling and no bathroom or kitchen to plumb.
B. Studio with a bathroom or kitchenette, still part of the house
A studio with its own bathroom, kitchenette, or both, that remains part of the existing house. It is not a second dwelling. Councils and certifiers look closely at whether a space with its own kitchen and bathroom is really still part of the house, so the layout and how the space connects to the house matter.
C. Secondary dwelling or granny flat
A self-contained dwelling occupied as a second dwelling on the lot. This is the biggest job of the three and the one most likely to need a planning application.
What job C costs
We’ve put together the rough estimate below to show where the money tends to go, because a single lump-sum figure hides where you have room to move. It is a guide, not a quote. It is based on job C, a 36m² double garage under the main roof, in a metropolitan area, converted to a self-contained granny flat with a bathroom and kitchenette. Your garage, your council, and your trades will move these numbers.
| Stage or trade | Low | High | Notes for owner-builders |
|---|---|---|---|
| Feasibility, survey, and site check | $900 | $2,400 | Ceiling height, slab condition, levels. Do this first, always. |
| Drafting and construction documentation | $2,800 | $6,500 | Change-of-use drawings are more detailed than a standard reno set. |
| Structural engineering | $1,200 | $3,800 | Lintel check, new footings, floor structure if raised. |
| Energy assessment (NatHERS, or BASIX in NSW) | $450 | $900 | Energy compliance applies to the new habitable space. The pathway differs by state. |
| Planning approval (DA, planning permit, or equivalent) | $1,100 | $4,200 | Fees vary widely by council and by estimated build cost. Where no planning application is needed, this line drops away. |
| Slab remediation or new floor structure | $3,000 | $14,000 | The single biggest variable in the whole job. |
| Demolition and garage door removal | $1,400 | $3,200 | Includes disposal. Asbestos is extra and changes this completely. |
| Masonry infill and new window openings | $3,800 | $9,500 | Lintels, brickwork, render or cladding to match. |
| Framing, insulation, and internal linings | $8,500 | $19,000 | Includes wall furring on masonry, ceiling, cornice, set and sand. |
| Electrical (new circuits, smoke alarms, switchboard) | $4,200 | $11,000 | A separate dwelling often needs a board upgrade. |
| Plumbing and drainage | $5,500 | $16,000 | Distance to the sewer connection point drives this. |
| Bathroom fit-out and waterproofing | $9,000 | $22,000 | Certified waterproofing only. No shortcuts here. |
| Kitchenette | $4,500 | $14,000 | Flat-pack to custom joinery. |
| Flooring, paint, and finishes | $4,000 | $11,000 | Where owner-builders genuinely save money. |
| Building certification and final sign-off | $1,600 | $3,500 | Multiple mandatory inspection stages. |
| Subtotal | $51,950 | $141,000 | |
| Contingency at 12 percent | $6,234 | $16,920 | On existing-structure work, 12 percent is the floor, not the ceiling. |
| Total | $58,184 | $157,920 | 36m² double garage, job C, self-contained granny flat |
The arithmetic. $51,950 plus 12 percent is $58,184. $141,000 plus 12 percent is $157,920. Divided across 36m², that is about $1,616 per square metre at the low end and $4,387 per square metre at the high end.
The low column is a realistic best case on a sound slab with adequate height. The high column is every line at its top at once, which is what happens when the slab fails, the sewer run is long, and the fit-out is custom. Asbestos removal, roof raising, fire separation, external regrading, and street service upgrades are not in either column. They sit on top when they apply, and each is covered below.
The cheapest genuine conversion we have seen properly documented and certified came in at $31,000 for a 20m² single garage in regional Victoria, where the slab already had a membrane and the ceiling measured 2.62 metres. The most expensive was $147,000 for a 42m² double on the east coast’s inner suburbs that required a new suspended timber floor, three new footings, and a full re-roof to achieve fall away from the new wall line.
Same job description. Nearly five times the cost. Nobody advertises that spread.
What job B costs
A studio that stays part of the house uses most of the lines in the table above. What changes is the second-dwelling layer. Whether a planning application is needed depends on whether the studio is still part of the house, and the board upgrade that often comes with a separate dwelling may not be needed, although new wet-area circuits still load the board. The slab, height, level, insulation, light, and ventilation work is the same.
What job A costs
Job A is a different and lower-cost job, and the table above should not be used to price it. Drop the bathroom, the kitchenette, and most of the plumbing and the job changes shape. In the table above, those three lines alone account for $19,000 to $52,000 before contingency. A room of the existing house also often avoids a planning application and never needs fire separation between two dwellings. It still carries the class change, so slab moisture, ceiling height, floor level, insulation, energy, light, and ventilation all still apply.
The class change is the whole project
Ask any drafter or structural engineer what the hardest part of a garage conversion is and they will not say the kitchenette. They will say proving to a certifier that a slab poured in 1987 for a Holden can now legally hold a bed.
Class 10a covers private garages, carports, and sheds. No insulation requirement, no habitable ceiling height, no vapour barrier unless the slab is continuous with the house slab, no minimum natural light or ventilation, no energy rating. Class 1a covers dwellings and their habitable rooms, and every one of those requirements applies. For building approval, the change of use is the trigger, not the renovation. You can leave every brick in place and still be doing Class 1a work in the eyes of the code. That is true for all three end uses.
What the change actually demands, in practice.
Ceiling height. ABCB Housing Provisions 10.3.1 sets 2.4 metres for habitable rooms other than kitchens, 2.1 metres for kitchens, and 2.1 metres for corridors, bathrooms, laundries, and sanitary compartments. A separate kitchen or kitchenette space is measured against the 2.1 metre kitchen figure. The studio or bedroom it opens onto still needs 2.4 metres. Where the ceiling is raked or skillion, 10.3.1(1)(e) asks for at least 2.4 metres over two-thirds of the floor area of a habitable room, or 2.2 metres in an attic, and under 10.3.1(2) any part lower than 1.5 metres is left out when that floor area is worked out. So the lowest point of a raked ceiling is not automatically fatal. A flat ceiling gets no such allowance. Garages are frequently built at 2.2 to 2.4 metres, and building the floor up to fix a slab or level problem takes height away. Raising a roof costs $12,000 to $28,000 depending on truss configuration.
Slab moisture. Under Housing Provisions 4.2.8, a Class 1 slab-on-ground needs a vapour barrier beneath it, and NSW calls for a damp-proofing membrane. A Class 10 slab only needs one where it is continuous with a Class 1 slab. A detached garage slab was never required to have one, and an older attached garage slab may not have one either. The deemed-to-satisfy answer is a membrane under the slab, which you cannot get under a slab that is already poured. The two practical paths are a topical epoxy membrane system on top of the slab at $85 to $140 per square metre, or a new floating floor structure over the slab at $180 to $320 per square metre. Neither is an automatic deemed-to-satisfy option. Your certifier decides whether either one is accepted, typically as a performance solution. The floor structure option also eats your ceiling height.
Floor level against outside ground. Garage slabs are often flush with or below the driveway so a car can drive in. Class 1 construction typically needs the finished floor above the surrounding ground with the ground falling away from the building, commonly around 150mm for a new slab. Your certifier will confirm what applies to your garage. The fix is either raising the internal floor or cutting and regrading the external surface, at $3,500 to $11,000.
Insulation and energy compliance. Uninsulated masonry garage walls need internal furring, batts, and lining. Roof space insulation typically falls somewhere in the R4.1 to R6.0 range, and the required value varies by climate zone. The full package runs $90 to $160 per square metre of wall and ceiling area. Houses are rated under NatHERS. NCC 2022 lifted the benchmark for new houses to 7 stars, so anyone still quoting off 6-star assumptions is working from a superseded standard. How energy compliance is shown for a converted garage depends on the state. Victoria, Queensland, and Tasmania work from the NCC energy provisions, and NSW uses BASIX. Your energy assessor and certifier will confirm the pathway for your job.
Natural light and ventilation. Under Housing Provisions 10.5.1, a habitable room needs windows with a light-transmitting area of not less than 10 percent of the floor area, or roof lights of not less than 3 percent. Ventilation is a separate requirement under Part 10.6, and 10.6.2 asks for an openable ventilating area of not less than 5 percent of the floor area. The garage door opening usually becomes the new window and wall, which is convenient, but the lintel above it was sized for a garage door, not a masonry infill and window head. Engineering check, $800 to $1,600.
Fire separation. This depends on the end use. A garage becoming part of the existing house, job A or B, is still one dwelling, so there is no separating wall to build between the garage and the house. Fire requirements can still apply to an external wall that sits close to a boundary, and your certifier will price that risk for your site. A secondary dwelling, job C, attached to the house is a second dwelling, and the wall between the two needs fire-rated construction. Budget $4,000 to $9,000 for that on job C.
Every one of these has to be checked on every conversion to habitable use. Not every one costs money on every garage. None of them show up in a $38,000 quote.
Owner-builder checklist. Build your own version of that table against your actual garage and your actual end use before you talk to a single trade. Our owner-builder checklist runs the same line items with space for your own quotes, your own contingency, and the costs specific to your slab.
Stage by stage, start to finish
People underestimate the front half of a garage conversion and overestimate the back half. The building works run eight to fourteen weeks. How long it takes to get to the point where building works are legal depends mostly on whether the job needs a planning application.
Stage 1: Feasibility and internal survey, 1 to 2 weeks
An internal survey (measure-up) conducted by a surveyor and a condition check on the slab, walls, and roof. Ceiling height gets measured across the whole room, because on a raked ceiling the two-thirds rule decides the outcome and on a flat ceiling one low measurement can end the job. Slab level relative to external ground gets checked with a level, not by eye. If the site is on a slope or has drainage issues, get a contour survey at $1000 to $1,800 rather than guessing.
This is the stage owner-builders skip. It is also the only stage that can save you $40,000 by telling you to stop.
Stage 2: Concept and drafting, 2 to 4 weeks
End use, layout, wall positions, window sizes, wet area locations, and the structural approach to the floor and lintel. For a conversion, this is more constrained than a new build because the walls, roof pitch, and drainage runs are already decided. Good documentation at this stage prevents variations later, and variations on an existing structure are the expensive kind.
Stage 3: Consultants, 2 to 3 weeks (parallel)
Engineering, energy assessment, and if relevant, a bushfire attack level assessment. Run these in parallel with drafting, not after. If your block is in a designated bushfire or flood area, read our breakdown of the cost of building in bushfire & flood zones (BAL compliance) before you finalise window and cladding selections, because BAL ratings change the glazing spec and the cost with it.
Stage 4: Planning approval, where it applies, 6 to 16 weeks
Two approvals get confused here. Building approval covers the class change from Class 10a to Class 1a, and every conversion to habitable use needs it. That is the building permit or construction certificate at the start and the final sign-off at the end. Planning approval is a separate question about land use, and it is not automatic.
For job A, an extra room of the house, the answer is often building approval only. Planning comes in where an overlay or heritage control applies, or where the conversion removes a car space the property is required to keep.
For job B, a studio that stays part of the house, building approval is always needed. Whether planning is needed depends on whether the studio is still treated as part of the house.
For job C, a secondary dwelling, planning applies in most cases, but the pathway differs by state.
NSW. Building work runs through either a complying development certificate or a DA followed by a construction certificate. A complying development pathway exists for secondary dwellings, but it has setback and site coverage criteria that an existing garage often fails, so a DA is common for job C.
VIC. A planning permit can be needed where the conversion creates a second dwelling or changes the approved use. Since Amendment VC253 in December 2023, a small second dwelling of 60m² or less no longer needs a planning permit on most lots without flooding, environmental, or other special planning controls. It still needs a building permit, and it cannot be connected to reticulated gas. A 36m² double garage sits inside that size limit, so check the overlays on your title first.
QLD. Under the Planning Regulation 2017, a secondary dwelling sits within the dwelling house use, so many councils need only building and plumbing approval. A material change of use application or referral comes in where the local planning scheme, the zone, or an overlay calls for it. A bedroom that stays part of the same house is not a material change of use.
TAS. A secondary dwelling is processed as a permitted or discretionary planning application depending on the zone and the neighbour interface.
Six weeks is a clean run in a well-resourced council. Sixteen weeks is a council with a backlog, a request for further information, or a neighbour objection. Where planning applies, plan on twelve. Where it does not, this stage drops out of the timeline.
Stage 5: Building approval and pre-start, 2 to 4 weeks
Construction certificate or building permit, owner-builder permit if you are managing the trades yourself, and insurance. Every end use goes through this stage. The owner-builder permit process is not a formality, and the requirements differ meaningfully between states, which we cover in the Owner-builder permits by state walkthrough. Get your Owner-builder insurance & warranty in Australia sorted before the first trade arrives, not after.
Stage 6: Construction, 8 to 14 weeks
Demolition and slab works, then masonry and framing, then services rough-in, then lining, then wet areas where there are any, then fit-off. Mandatory inspections sit between most of those. The slab remediation stage is where delays cluster, because if the certifier does not accept the proposed moisture solution you are back to the engineer for a revised floor detail.
Stage 7: Certification and occupancy, 2 to 4 weeks
Final inspection, energy compliance verification, smoke alarm certification, and the final sign-off. The name of that document varies by state. NSW issues an occupation certificate. Victoria issues an occupancy permit or a certificate of final inspection, depending on what the building permit requires. Queensland issues a final inspection certificate, and Tasmania a certificate of completion. Without it the space is not legally habitable, which matters for insurance, for renting it out, and for the sale contract in five years.
Add the stages up and the timelines split by end use. For job C with planning, the stage ranges add to roughly 21 to 44 weeks, and with a typical twelve-week planning stage most jobs land at six to nine months. For job A with building approval only, feasibility, drafting, and building approval add to about five to ten weeks before construction, and the whole job runs roughly 15 to 28 weeks, or about four to six months. Job B sits at the job A end when no planning is needed and moves toward the job C end when it is. If anyone tells you four weeks, ask them which of those seven stages they are planning to skip. For a wider sense of how these durations behave across secondary dwelling projects, our stage-by-stage look at How long does it take to build a granny flat? stage-by-stage timeline breaks the same pattern down further.
Where the budget quietly blows out
The blowouts we see are not glamorous. Nobody goes over budget on tapware.
The first is asbestos. Garages built between 1950 and 1987 frequently have asbestos cement sheeting in the walls, ceiling, or eaves. Licensed removal runs $60 to $140 per square metre, plus air monitoring and clearance certificates. A modest double garage with sheeted walls and an asbestos ceiling can add $8,000 to $18,000, and it is not negotiable. This applies to all three end uses.
The second is the sewer connection, which matters for jobs B and C. A garage on the far side of the block from the house’s drainage line is a plumbing problem dressed up as a location. Every additional metre of sewer run through a driveway or established landscaping costs money, and if the fall is wrong you are looking at a pump-out system at $6,500 to $14,000. We have seen the plumbing line alone exceed the entire internal fit-out.
The third is the electrical board. Converting a garage into a self-contained dwelling frequently pushes the existing switchboard past capacity, especially on properties still running an older main. A board upgrade and new sub-main is $3,500 to $9,000. If the network operator requires a service upgrade at the street, add $4,000 to $12,000 and four to eight weeks. This is mostly a job C problem, although new wet-area circuits on job B can trigger it too.
The fourth is the roof. Garages under the main house roof are usually fine. Freestanding garages with a low-pitch skillion and no eaves often fail on both ceiling height and weatherproofing at the same time, and the cheapest compliant answer is a new roof structure rather than patching the old one.
None of these appear in a per-square-metre estimate. They appear in week six.
How to keep the number under control
The controllable costs on a conversion are not the ones people try to control.
Decide the end use first. An extra bedroom, a studio that stays part of the house, and a granny flat are priced, approved, and timed differently, and changing your mind halfway through the drawings costs money.
Measure the ceiling height and the slab level before you do anything else. Two measurements, twenty minutes, and they determine roughly 40 percent of your final cost. If the ceiling sits under 2.5 metres across much of the room, assume you have a problem and budget for it rather than hoping, because any floor build-up to fix the slab or the level comes straight out of that height.
Keep the wet areas close to the existing drainage. Every metre you move a shower away from the nearest connection point adds cost and adds fall problems. A layout that puts the bathroom on the wall nearest the house’s sewer line will beat a prettier layout on price every single time.
Do the work you are actually competent to do, and hire out the rest. Painting, flooring, landscaping, and demolition strip-out are genuine owner-builder savings, often $8,000 to $15,000 on a job this size. Waterproofing, electrical, and structural work are not, and our guide to Granny Flat DIY in Australia sets out that line clearly.
If you are weighing a job C conversion against a new granny flat, compare them honestly. On a 36m² double garage with a bad slab and a low ceiling, we have costed conversions at $2,900 per square metre against a purpose-built detached studio at $2,400 per square metre. The new build was cheaper, better insulated, and easier to approve. Look at the 2 Bedroom Granny Flat Prices in Australia and your local numbers in Granny Flat Cost Sydney, Granny Flat Cost Melbourne, or Granny Flat Cost Brisbane before you commit to the conversion on sentiment alone.
Sometimes the garage is worth keeping as a garage.
What Draftee does. We draw the plans. We do not sell buildings. That distinction matters more on a garage conversion than almost any other job, because the company quoting you a fixed price has a commercial interest in the slab being fine, the ceiling being tall enough, and the class change being straightforward. We do not. If the feasibility shows your garage cannot reasonably carry a Class 1a classification without $40,000 of structural work, that is what our drawings will show, and the decision stays yours. We have no product to move. We produce the documentation, the layouts, and the compliance set for council and certifier assessment, and you take that to whichever builders or trades you choose. Our incentive is getting the drawings right the first time, because revisions cost us and delays cost you.
Continue Your Research
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Last updated and changelog
Last updated: 17 September 2026
Changelog:
– 17 September 2026: Initial publication
