house extension costs: what australian builders need to budget
Extensions are the big-ticket jobs. They are also where the biggest money gets lost. Council approvals drag on, engineers find problems, the existing structure does not line up with the plans. Every week of delay costs you labour, hire fees, and opportunity cost on the next job.
This page breaks down real extension costs in Australia in 2026 and flags where the risk hides on each stage of the build.
cost per square metre
The most common way to ballpark an extension in Australia is cost per square metre. These are builder-to-client rates including labour, materials, and margin — but excluding council fees, engineer reports, and site costs.
| Extension Type | Cost / sqm | Risk Level | Notes |
|---|---|---|---|
| Single-storey rear extension | $2,000 - $3,500 | Medium | Standard timber or steel frame. Slab or stumps. |
| Double-storey extension | $2,800 - $4,500 | High | Structural steel, engineering, scaffolding costs. |
| Granny flat / separate dwelling | $1,800 - $3,200 | Medium | Separate slab. Simpler than tying into existing structure. |
| Sunroom / enclosed alfresco | $1,500 - $2,800 | Low - Medium | Lighter build. Insulation and glazing drive cost. |
worked example: 30 sqm single-storey rear extension
Adding a living area and second bathroom to a 1970s brick home in Melbourne. Timber frame extension on a concrete slab, tying into the existing roofline with a Colorbond skillion roof.
| Item | Cost Range | Risk Level | Notes |
|---|---|---|---|
| Council / planning approvals | $2,000 - $5,000 | High | Heritage overlays, neighbour objections, delays |
| Engineer & drafting | $3,000 - $6,000 | Medium | Structural engineer, energy report, soil test |
| Site prep & demolition | $2,000 - $4,000 | Medium | Removing existing walls, stumps, or slab sections |
| Concrete slab | $4,500 - $7,000 | Low - Medium | Standard waffle pod or raft slab. Reactive soil = more cost. |
| Framing (timber or steel) | $6,000 - $10,000 | Medium | LVL beams for openings. Steel portal frames push this up. |
| Roofing (Colorbond) | $3,500 - $6,000 | Low | Tying into existing roofline adds complexity |
| Windows & external doors | $3,000 - $6,000 | Medium | Aluminium standard = Low. Bifold/stacker doors = High. |
| Plumbing (new bathroom) | $4,000 - $7,000 | Medium | New wet area. Sewer connection can be tricky on older blocks. |
| Electrics | $3,000 - $5,000 | Medium | New circuits, switchboard upgrade if needed |
| Plastering & internal linings | $3,000 - $5,000 | Low | Standard plasterboard. Cornice and finishing. |
| Tiling (bathroom & laundry) | $2,500 - $4,500 | Low - Medium | Including waterproofing to Australian Standards |
| Painting | $2,000 - $3,500 | Low | Interior and exterior to match existing |
| Flooring | $2,000 - $4,000 | Low | Hybrid plank or tiles. Subfloor is new so minimal prep. |
Total range for a 30 sqm extension: $40,500 - $77,000. That is roughly $1,350 - $2,570 per square metre all-in. The council approvals and engineering alone can swing things by $6,000 before you even break ground.
where extensions blow budgets
- Council delays. In some councils, a Development Application can take 3-6 months. Every month of delay is a month you are not billing. Factor this into your programme and cashflow — or make sure the customer covers holding costs.
- Reactive soil. Much of Melbourne, Adelaide, and parts of Sydney sit on reactive clay. This means deeper footings, more reinforcement, and potentially engineered slab designs. Get a soil test before pricing the slab.
- Tying into existing structure. The junction between old and new is where cracks appear, roofs leak, and levels do not match. Budget extra time and materials for the connection point. This is Medium uncertainty at minimum.
- Scope creep from the client. "While you are opening up the back, can we redo the kitchen too?" Extensions attract massive scope creep. Get the full scope locked down in writing before you start.
quoting extensions smarter
The slab, framing, and roofing are relatively predictable for experienced builders. The council approvals, engineering discoveries, and connection to the existing structure are where the uncertainty lives. Rate each stage honestly, simulate the range, and quote at P80. On a $60,000 extension, the difference between a flat 10% contingency and a risk-weighted one could be $5,000 or more.