How Much Does a Home Extension Cost in Sydney?
On this page14 sections
- Home extension costs in Sydney by size
- What the per square metre rate covers
- Preliminaries: the costs that come with every job
- Six things that move your extension price
- Where extensions go wrong: the junction
- Why extension quotes vary so much
- What NSW rules say about paying for an extension
- Approvals and timeframes
- Extend out or build up?
- How to get an accurate extension price
- When an extension is the wrong move
- Before you get extension quotes: checklist
- Home extension cost FAQs
- Sources and references
Every extension conversation starts with “we just want to push the back wall out a bit.” The words “a bit” are doing a lot of work in that sentence. This guide is for owners who want the real number before they pay for drawings: price ranges by size, what the rate covers, the six things that move it, and why two quotes for the same job can look so different.
Home extension costs in Sydney by size
These ranges are our published rate of $2,500 to $4,500 per m² applied to common extension sizes. They’re build cost only, before preliminaries.
| Extension size | Typical use | Build cost range | Plus preliminaries |
|---|---|---|---|
| 20m² | Bigger family room, or a bedroom with ensuite | $50,000 to $90,000 | Around $30,000 |
| 30m² | Open-plan kitchen and dining across the back | $75,000 to $135,000 | Around $30,000 |
| 45m² | Kitchen, family room and a laundry | $112,500 to $202,500 | Around $30,000 |
| 60m² | Living wing, or two bedrooms and a bathroom | $150,000 to $270,000 | Around $30,000 |
Why does the same 30m² land anywhere between $75,000 and $135,000? Mostly because of what’s in it and what it joins onto. A 30m² rumpus room with a timber floor is the cheap end. A 30m² kitchen with a stone island, a butler’s pantry and a 6 metre stacker door is not. The kitchen is the most expensive room per square metre, and also where everyone stands at parties anyway, so most people decide it’s worth it.
One more thing on dates. House construction prices in NSW rose 5.2% in the year to September 2026 (ABS Producer Price Index). If the extension price you’ve got in your head came from a neighbour who built in 2021, add a bit. Then add a bit more.
What the per square metre rate covers
The rate covers building the new space and tying it into the house:
- Footings and slab, or a suspended floor on a sloping block, engineered for your soil
- Wall and roof framing, roofing and the roof tie-in to the existing house
- Windows and external doors, cladding or brickwork, insulation and plasterboard
- Internal finishes, electrical and lighting in the new area
- The opening into the existing house, and re-bracing the existing structure around it
- Wet areas inspected before tiling, to AS 3740
We photograph the work before it’s covered and hand that record over at completion, so you know what’s behind your new walls.
Preliminaries: the costs that come with every job
Preliminaries add around $30,000 on top of the build cost. They cover design, engineering and council approvals, plus site set-up like fencing and temporary services. A 20m² extension and a 60m² extension both need drawings, an engineer and a fence, which is why small extensions cost more per square metre than big ones.
Design and council approval come before any of this, and the pathway changes the timeline more than the price. See our guide on DA vs CDC in NSW.
Six things that move your extension price
- The opening into the house. Taking out a rear wall to open the kitchen onto the new room usually means a steel or engineered beam, temporary propping and an engineer’s design. It’s often the single biggest line in the quote.
- Wet areas. A kitchen, bathroom or laundry in the extension adds plumbing, drainage, waterproofing and tiling. A dry family room has none of that.
- How much of the existing house changes. Matching floor levels, reworking the roof where old meets new, and moving walls in the existing house all add up fast.
- The ground. Reactive clay, common across Western Sydney, needs deeper footings. We recommend piers taken down to shale or rock, which costs more and saves a lot of cracked plaster later. Our How We Build page explains why.
- Site access. If the only way to the backyard is a 900mm side path, every wheelbarrow of concrete, every sheet and every brick goes in by hand.
- Windows, doors and finishes. Big stacker doors, double glazing, a raked ceiling or premium cladding move the number up. Standard aluminium windows and a simple roof keep it down.
Where extensions go wrong: the junction
Our opinion, after a lot of extensions: the junction between old and new is the cheapest part to do properly and the most expensive part to fix later. It’s where cracks show up, where roofs leak and where floors don’t line up. We detail it, re-brace the existing structure and engineer the footings for the soil. Our post on why extensions fail at the junction goes through it in detail.
Why extension quotes vary so much
Two quotes for the same extension can be tens of thousands of dollars apart and both be honest. Usually they’re pricing different things. Line them up on the same basis before you compare totals:
- Prime cost (PC) items: allowances for things you haven’t picked yet, like tapware, tiles or the oven. A $3,000 tile allowance and a $9,000 tile allowance make very different totals.
- Provisional sums: allowances for work nobody can price until it’s opened up, like the footings under an old rear wall. Low provisional sums make a quote look cheap and come back as variations.
- Demolition and preparation: is removing the old laundry, propping the roof and making good the existing walls in the price?
- Structural work: is the beam over the new opening priced, or “subject to engineer”?
- Preliminaries: design, engineering, approvals and site set-up. In, or quietly left out?
- Exclusions: read them twice. It’s the most honest part of any quote.
What NSW rules say about paying for an extension
- Deposit: a builder can ask for 10% of the contract price at most.
- Home Building Compensation cover: required for any job over $20,000, taken out by the builder before any deposit is asked for.
- Progress payments: must match work done, not weeks on site.
- Cooling-off: five clear business days on contracts over $20,000.
Full details are on the NSW Government’s home building contracts page. When you compare quotes, check every one says whether GST is included.
Approvals and timeframes
- Approval: Many Sydney extensions qualify for a CDC through a private certifier, which is much faster than a DA. Heritage listings and conservation areas, such as Hunters Hill, Beecroft and parts of the inner west, rule the CDC option out.
- Design and approvals: a CDC takes about 4 to 8 weeks; a DA can take up to 6 months, depending on council.
- Construction: 12 to 20 weeks on site for most extensions.
- Living at home: Usually possible. The messiest stage is the breakthrough into the existing house, and we plan that with you in advance.
Extend out or build up?
Going out is usually cheaper: $2,500 to $4,500 per m² at ground level against $3,500 to $5,500 per m² for a second storey. Going up makes sense when the yard is small, the land is steep at the back, or the backyard is the reason you bought the house. Our second storey cost guide runs the numbers for going up. The Extend or Build Up tool compares both for the same space.
How to get an accurate extension price
- Answer a few questions in our consultation questionnaire.
- A builder calls within 24 to 48 business hours, then walks your home and yard with you.
- You get an itemised written estimate. If the project doesn’t stack up, we tell you.
- Approval pathway confirmed before quoting, so the timeline in the quote is real.
Tool: Home Extension Cost Calculator. Get a range for your extension by size, rooms and finish.
Your block changes the job too. On the bigger Hills blocks in Castle Hill, there’s usually room for machines and a crane, which keeps labour down. In parts of Western Sydney like Smithfield, sewer mains often cross the backyard, and building near one needs Sydney Water approval before the design is locked. How we work and the areas we cover are on our home extensions Sydney page.
When an extension is the wrong move
- The yard is your only outdoor space. A 30m² extension on a small lot can leave you a house with no garden. Going up keeps it.
- What you need is a better layout. If the house is big enough and badly arranged, a home renovation inside the existing footprint can fix it for less.
- The house needs major structural work first. Sometimes the honest answer is a knockdown rebuild, and our custom homes page covers that.
Before you get extension quotes: checklist
- Rough size, and what goes in it: kitchen, family room, bathroom, laundry
- How wide the opening into the existing house needs to be
- Side access width, and where a skip can go
- Your planning certificate: any heritage, flood or bushfire notes
- Whether a Sydney Water sewer line runs through the backyard (a sewer service diagram shows it)
- Known cracks or movement in the existing house
- Your budget range, including preliminaries
- The same list of inclusions for every builder you ask, so the quotes compare
Home extension cost FAQs
Between $70,000 and $300,000 for most projects, depending on size and how much of the existing house changes. Ground level work runs at $2,500 to $4,500 per m², plus around $30,000 in preliminaries.
$2,500 to $4,500 per m² for ground floor extension work. Kitchens, bathrooms and big structural openings sit at the top of that range.
Because fixed costs like design, engineering and council approvals, plus site set-up, are spread over fewer square metres.
Not always. Many Sydney extensions can be approved as a CDC by a private certifier. Heritage items and conservation areas need a DA.
Most take 12 to 20 weeks on site, plus design and approvals beforehand.
Extending out is usually cheaper per square metre. Going up costs more but keeps your yard.
Want a price for your extension? Request a consultation or call Stav on 0450 605 080, Monday to Friday, 7am to 5pm.
Sources and references
- Australian Bureau of Statistics: Producer Price Indexes, Australia
- NSW Government: Home building contracts
- NSW Planning Portal: Housing Code
- Strava Group: Foundations and slabs and how we waterproof wet areas
