How Much Does a Home Extension Cost in Australia?
Real 2026 numbers for Sydney, what each square metre actually buys, and the approval costs most builders leave out of the quote.
Prices checked 23 October 2026. Ranges are for Sydney metro, mid-range finishes, supply and build, including GST.
What an extension costs in 2026
Most home extensions in Sydney land between $2,500 and $4,500 per square metre. What moves you up that scale is not the carpet. It is how much structure the job needs: new footings, steel, a roof, and whether the new work has to match a house that is already standing.
| Job | Typical range | What you get |
|---|---|---|
| Ground floor rear extension, about 20 sqm | $50,000 to $90,000 | Open plan living or dining addition, new slab, new roof junction, sliding doors, floor and ceiling finishes matched to the existing house |
| Ground floor extension, about 40 sqm | $100,000 to $180,000 | Kitchen, living and dining extension, often with structural steel where the old rear wall comes out |
| Second storey addition, about 40 sqm | $140,000 to $240,000 | Two to three bedrooms plus a bathroom upstairs, new stairs, upgraded footings, new roof or roof conversion |
| Full upper floor with roof replacement | $180,000 to $300,000 | Whole new level including roof removal and rebuild, the most structural version of an extension |
| Wrap around or multi side extension | $4,000 to $6,500 per sqm | More external wall, more corners, more roof junctions, which is why the rate climbs |
| Premium architectural extension | $6,000 to $9,000 per sqm | Full height glazing, custom joinery, stone, heated floors, engineered timber, high end detailing throughout |
| Attached secondary dwelling, about 60 sqm | $120,000 to $220,000 | Self contained one or two bedroom flat with its own kitchen, bathroom and entry |
As a sanity check, a 25 square metre extension at $3,200 per square metre is $80,000 of building work. Add approvals and consultants on top, which the next two sections cover, because they are not in that number.
Where the money goes
| Stage | Share of the build | What it covers |
|---|---|---|
| Site preparation and demolition | 4 to 8% | Removing the old wall, roof or paving, skip bins, protecting the rest of the house |
| Footings and slab | 8 to 14% | Excavation, piers, slab or bearer and joist floor. More on a slope or on reactive clay |
| Frame and structural steel | 10 to 18% | Timber frame, beams and posts where a wall is opened up. Steel is priced by the kilo and it is not cheap |
| Roof and roof junction | 6 to 10% | New roof framing, tiles or metal, flashings and gutter where the new roof meets the old one |
| Windows, doors and glazing | 6 to 12% | Sliding stacker doors, bifolds, double glazing. The single fastest way to add 20,000 dollars |
| External cladding and brickwork | 8 to 14% | Matching existing brick, render, weatherboard or panel, plus insulation and sarking |
| Electrical | 4 to 8% | New circuits, downlights, power, smoke alarms, data and switchboard capacity |
| Plumbing and drainage | 4 to 8% | Higher if a bathroom or kitchen moves, or if stormwater needs a new connection or tank |
| Insulation, plaster and floors | 5 to 10% | Ceiling and wall linings, cornice, insulation, screed, timber or tile flooring |
| Kitchen, bathroom or laundry fitout | 10 to 20% | Only where the extension includes a wet area or joinery. Cabinetry is the biggest line in it |
| Painting and finishes | 5 to 9% | Paint inside and out, trims, door hardware, silicone and make good |
| Builder margin and supervision | 15 to 25% | Project management, insurances, warranty, co ordination of trades, and the site clean |
The costs that sit outside the build quote
This is the part that catches people. A build quote covers building. It rarely covers the paperwork that makes the building legal, and that paperwork runs into the tens of thousands on a large extension.
| Item | Typical cost | Notes |
|---|---|---|
| Survey | $1,500 to $3,500 | Needed for any proper drawing set, and for levels and boundaries |
| Architect or building designer | 5 to 10% of build cost | Or a fixed fee, commonly $6,000 to $20,000 for a documented extension depending on complexity |
| Structural engineer | $1,500 to $4,500 | Footings, beams, lintels. More where a wall is removed or a level is added |
| Council or certifier fees | $1,000 to $6,000 | Complying development is usually cheaper and faster than a full development application |
| BASIX certificate | $500 to $1,500 | Required for most residential alterations and additions above $50,000 of work in NSW |
| Stormwater and drainage works | $2,000 to $10,000 | On site detention, new pits and lines, connections. Often a condition of consent |
| Home building compensation cover | Premium on the contract value | Required for residential work over $20,000. Ask for the certificate before work starts |
Budget 10 to 15 per cent on top of the build quote for consultants and approvals. On an $80,000 extension that is another $8,000 to $12,000, and it gets spent before a single tradie arrives.
What pushes the price up
- Removing a wall and adding steel. A single steel beam can add $8,000 to $25,000 once you count the engineering, the crane or the crew to lift it, and the plaster and paint to make it look like it was never there.
- Glazing. Bifold and stacker doors are priced per panel, and double glazing roughly doubles the glass line. This is the most common cause of a 20 per cent blowout.
- Matching an existing house. Old brick, old roof tiles, old cornice profiles. Matching costs more than starting again, and on heritage or conservation areas it is often mandatory.
- A sloping site. Cut and fill, retaining walls, steps and drainage. A site that falls two metres across the block can add tens of thousands before the slab is poured.
- Asbestos. Homes built or renovated before 1990 can have asbestos sheeting in eaves, wet areas and fences. It has to be tested and removed by a licensed contractor, and that is a real line item.
- Trees and root zones. A protected tree within the zone of influence can dictate where the extension can sit, and sometimes rules it out entirely.
- Bringing the old house up to code. New work can trigger upgrades to smoke alarms, glazing, insulation, stairs and balustrades in the parts you were not planning to touch.
The approval path in NSW
There are two doors, and which one you use decides how long the job waits before it starts.
Complying development, the fast door
If the extension meets the code limits for setbacks, height, floor space ratio, landscaping and drainage, a private certifier can issue a complying development certificate, usually in about three to four weeks. No council meeting, no neighbour notification, no design changes forced by a panel.
Development application, the slow door
Anything that does not fit the code goes to council. Expect three to six months for a straightforward DA, longer if there is a heritage item, a flood overlay, a bushfire zone or an objection from a neighbour. This is the stage where a project loses a year, and it is worth paying a designer who knows the local rules before you buy a single tile.
Either way, check before you buy: a surveyor or your certifier can tell you in a week whether the plan you have in mind fits the code. That week saves months.
The five things that go wrong
- A quote built on allowances. "Allowance" and "PC sum" mean the price is not fixed. Ask what the allowance buys and what happens when it runs out, because it will run out.
- Comparing quotes on different scopes. The cheapest quote is usually the one with the most left out: no stormwater, no make good, no painting, no asbestos allowance.
- A deposit bigger than 10%. Under NSW law a deposit for residential building work cannot exceed 10 per cent of the contract price. Anyone asking for 30 per cent is using you as a bank.
- No written contract. Residential building work over $5,000 needs a written contract with the consumer notes attached.
- No insurance above $20,000. Work over $20,000 needs home building compensation cover. If the builder cannot produce the certificate, do not start.
All five are checkable in ten minutes. Our NSW licensed trades guide shows exactly where to look, and our bathroom, kitchen and deck cost guides cover the rooms inside that extension.
The short version
Work out the square metres you actually need, multiply by $2,500 to $4,500, then add 10 to 15 per cent for approvals and consultants, then add a contingency. If the total still makes sense next to the cost of moving, get three written quotes on the same scope and compare line by line, not total to total.
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Common questions
How much does a home extension cost in Australia in 2026?
A ground floor rear extension in Sydney typically runs 2,500 to 4,500 dollars per square metre, so a 20 square metre addition is about 50,000 to 90,000 dollars. A second storey addition usually costs 3,500 to 6,000 dollars per square metre because of the structural work and the stairs. Approval and consultant costs sit on top of the build quote.
Is it cheaper to extend or to move?
In most Sydney suburbs, extending is cheaper than selling and buying a bigger house, because stamp duty alone on a 1.5 million dollar purchase is over 60,000 dollars, before agent fees and moving costs. Extending also keeps the street, the school run and the neighbours.
How long does a home extension take to build?
Twelve to twenty weeks on site for a straightforward single storey extension, longer if the roof is being altered or a second storey goes up. Add six to ten weeks for design and documentation, then three to six weeks for a complying development certificate or three to six months for a full development application.
Do I need council approval to extend a house in NSW?
Not always. Many single and two storey extensions qualify as complying development and can be approved by a private certifier in about three to four weeks, provided they meet the setbacks, height and floor space limits in the code. Anything outside those limits needs a development application through council.
What is not included in a builder's extension quote?
Usually the survey, the architect or designer, the structural engineer, council or certifier fees, BASIX, and stormwater work. Get all of those quoted separately in writing before you sign, and keep ten to fifteen per cent of the build cost aside for the things that only turn up once the walls are open.