What our analysis shows
Analysis by Granny Flat Numbers, data as of 23 October 2026
- Our estimate for turning a 36 m² double garage into a one-bedroom flat in Sydney is $85,500, or $97,500 with a replacement carport. A new 36 m² detached flat costs $157,000 in our model, so the conversion saves roughly 38%.
- The saving comes only from the structure you keep. The fit-out, sewer connection, design, approval and council contribution cost about the same as in a new flat, and they make up 81% of the conversion bill.
- At Sydney's median detached one-bedroom rent of $550 a week, the converted garage pays back in 5.7 years after tax, against 9.1 for a new flat of the same size.
- Parking is the hidden cost. A Victorian owner in 2024 was told by the building surveyor to provide two replacement car spaces, although the flat itself needed none. In NSW, Ryde council requires at least one off-street space to remain for the main house.
Where the money goes in a double-garage conversion
- Kitchen, bathroom, linings, electrical and plumbing fit-out$41,000
- Replacement carport for the main house$12,000
- Insulation, windows and doors in the existing walls$10,500
- Sewer and water connection$10,000
- BCA report, design and BASIX/energy assessment$8,000
- Floor: damp-proofing and raising to habitable level$5,900
- Council s7.11 contribution (typical)$5,500
- Approval: Complying development certificate (CDC)$4,500
Our estimate for a 36 m² attached or detached double garage in Sydney, standard finish, structure sound and kept.
Our cost estimate
We built the estimate from our new-build model. A new flat's construction cost splits roughly into structure, envelope and interior fit-out. Converting a sound garage keeps the structure and most of the envelope, so we priced the fit-out (35% of the new-build construction cost), part of the envelope (9%) and a floor fix (5%). Connections, design, approval and council contribution are priced as for a new flat.
| Line | Single garage, 18 m² | Double garage, 36 m² |
|---|---|---|
| Kitchen, bathroom, linings, electrical and plumbing fit-out | $21,200 | $41,000 |
| Insulation, windows and doors in the existing walls | $5,400 | $10,500 |
| Floor: damp-proofing and raising to habitable level | $3,000 | $5,900 |
| Sewer and water connection | $10,000 | $10,000 |
| BCA report, design and BASIX/energy assessment | $8,000 | $8,000 |
| Approval: Complying development certificate (CDC) | $4,500 | $4,500 |
| Council s7.11 contribution (typical) | $5,500 | $5,500 |
| Total | $57,500 | $85,500 |
Owners' quotes on forums sit around these figures. One 2023 Sydney owner was quoted $30,000 just to turn half a double garage into a bedroom, with no bathroom or kitchen. Another saw a conversion quote double within six months during the 2022–23 cost rises.
What the building code requires
A garage is a Class 10a non-habitable building. As a granny flat it becomes a Class 1a dwelling, and has to meet the code for one. Ryde council's guideline lists what a BCA report on a conversion has to cover:
- The floor. A habitable slab must be at least 150 mm above the adjacent ground with a damp-proof membrane underneath. Most garage slabs fail this, which means a raised floor or a membrane system on top.
- Ceiling height. 2.4 m in habitable rooms, 2.1 m in kitchens and bathrooms.
- Termite protection for the new habitable use.
- Light and ventilation. Windows sized to the floor area of each room, which usually means new openings in the garage walls.
- Setbacks. Walls at least 900 mm from the boundary; many garages sit closer.
- Energy. In NSW a BASIX certificate; elsewhere the NCC energy provisions, which mean insulation in walls and ceiling.
If the garage is attached to the house, the shared wall needs fire separation as between two dwellings. Ryde's guideline also requires 24 m² of private open space for the flat and direct access from the street. Source: City of Ryde: guidelines for converting a garage to a secondary dwelling.
The parking catch
The garage was the house's parking. Planning rules in most councils require the main house to keep at least one off-street space, and some ask for more. Victoria's small second dwelling rules require no parking for the flat itself, but that does not remove the house's own requirement: in a 2024 forum thread a Melbourne owner was told to find two replacement spaces on the lot. Budget for a carport or hardstand, or confirm in writing that the existing driveway counts.
Does it pay?
| Sydney, 1 bedroom, rent $550/wk | Cost | Net yield | Payback |
|---|---|---|---|
| Garage conversion with new carport | $97,500 | 24.0% | 5.7 yrs |
| New 36 m² detached flat | $157,000 | 14.7% | 9.1 yrs |
A conversion pays back faster when the garage is sound, high enough and well away from the boundary. It stops paying when the slab, ceiling height or setback has to be rebuilt: at that point you are paying for a new building inside an old shell, and a new detached flat with its own yard usually rents better.
Check before you start
- Ceiling height inside the garage, and the slab level against the ground outside.
- Distance from each wall to the boundary.
- Where the sewer is, and whether the new bathroom can drain to it by gravity.
- Whether the house keeps a legal car space.
- If the garage was converted by a previous owner without approval, read unapproved granny flats first.
This week's worked example
Week of 19 October 2026. A new example, rule and owner report every Monday.
A 55 m² two-bedroom flat with a premium finish on a block with some slope, services 12 m from the house, in Whittlesea: our model puts it at $243,000 turnkey ($214,000–$272,500), or $4,420 per m². At the area's median 2-bedroom bond rent of $480 a week it would return 8.0% net and pay back in 16.1 years after tax.
Rule check: Builder deposits are capped by law: 10% in NSW, 5% in Victoria and Queensland for contracts of $20,000 or more, and 6.5% in Western Australia.
From an owner (NSW, 2025): A family found through a council records search that their 1970s granny flat, rented out for decades, only ever had interim approval as a workshop.
Questions people ask
How much does it cost to convert a garage into a granny flat?
About $85,500 for a double garage in Sydney in our estimate, plus around $12,000 if you need to replace the car space. A single garage (18 m²) comes to about $57,500, because the kitchen, bathroom and approvals cost the same.
Do I need approval to convert a garage into a granny flat?
Yes. The building changes from a non-habitable Class 10a structure to a Class 1a dwelling, so it needs approval under the planning rules and must meet the building code for a dwelling, including damp-proofing, ceiling height, light and ventilation, and energy efficiency.
What ceiling height does a converted garage need?
The building code sets 2.4 m for habitable rooms such as bedrooms and living areas, and 2.1 m for kitchens, bathrooms and laundries. Many garages are lower, which can mean lowering the floor or raising the roof.