If you’ve typed “how much does a house extension cost” into Google at 11pm with a cup of tea going cold beside you, you’re not alone. It’s one of the first questions every homeowner asks before they even pick up the phone to a builder and fair enough, because an extension is one of the biggest financial decisions you’ll make outside of buying the house itself.
The honest answer? A house extension in Australia typically costs between $2,500 and $5,000 per square metre, with most single-storey additions landing somewhere between $80,000 and $250,000 depending on size, finish level, and location. A second storey addition usually costs more per square metre, often $3,000 to $5,500 because you’re dealing with structural upgrades, roof removal, and temporary accommodation while the roof’s off.
That’s the quick answer. But if you want to actually budget properly (and not get blindsided three months into the build), stick with me we’ll break down exactly what drives that house extension cost, what it looks like city by city, and where you can genuinely save money without regretting it later.
Quick Answer: Average House Extension Cost Per m²
| Extension Type | Cost Per m² | Typical Total Cost |
|---|---|---|
| Basic single storey extension | $2,000 – $2,800 | $80,000 – $150,000 |
| Mid-range single storey extension | $2,800 – $3,800 | $120,000 – $200,000 |
| High-end single storey extension | $3,800 – $5,000+ | $180,000 – $300,000+ |
| Second storey extension | $3,000 – $5,500 | $200,000 – $400,000+ |
| Rear extension (kitchen/living) | $2,500 – $4,000 | $100,000 – $220,000 |
| Granny flat / small self-contained addition | $1,800 – $3,000 | $90,000 – $180,000 |
These figures reflect national averages as of 2026. Prices shift depending on your state, your block, and how fussy (in a good way) you get with finishes.
What Actually Drives the Cost of a House Extension?
Every builder gets asked “why does it cost that much?” at least once a week, and it’s a fair question. Here’s what really moves the needle on your house extension cost:
1. Size of the Extension (H3 Tag)
This one’s obvious but worth spelling out the bigger the footprint, the higher the total cost, even though the cost per m² often drops slightly on larger builds because fixed costs (site setup, council approvals, site sheds) get spread across more floor area. A 20m² extension might cost more per square metre than a 60m² one for exactly this reason.
2. Single Storey vs Second Storey Extension (H3 Tag)
A single storey extension cost is generally lower because you’re not touching the existing roof structure, load-bearing walls, or adding a staircase. A second storey extension (also called a double story extension) is a different beast entirely you’re often removing the whole roof, reinforcing the existing ground floor to carry the new load, and living through a much longer, messier build. Expect to pay 30–50% more per square metre for a second storey compared to a ground floor addition.
3. What You’re Building (H3 Tag)
A bathroom or laundry extension (heavy on plumbing, waterproofing, tiling) costs more per square metre than a simple bedroom addition. A kitchen extension sits somewhere in between it depends heavily on the cabinetry and appliance package you choose. If you’re combining structural work with a full kitchen renovation, that’s a separate budget line worth planning for early.
4. Site Access and Existing Structure (H3 Tag)
Narrow side access, sloping blocks, heritage overlays, or an older weatherboard or Federation-era home needing extra structural work will all push your house extension cost up. A weatherboard house extension matched to existing cladding, for example, often costs more than a straightforward brick veneer match because of the labour involved in blending old and new.
5. Finish Level (H3 Tag)
Standard fixtures and fittings versus a fully custom, architect-designed fitout can swing your final number by tens of thousands of dollars. Stone benchtops, engineered timber flooring, double-glazed windows, and custom cabinetry all add up fast but they also add resale value, so it’s worth weighing cost against long-term return.
6. Council Approvals and Planning (H3 Tag)
Depending on your council and whether your extension needs a full Development Application (DA) or can go through as Complying Development, approval costs and timeframes vary. Heritage-listed suburbs, bushfire zones, and flood-prone areas typically mean extra reports (structural, bushfire, stormwater) that add to both cost and time.
1. Size of the Bathroom
A standard Australian bathroom is somewhere between 4 and 7 square metres. Squeeze into a tiny powder room and you’ll naturally spend less. Stretch out into a big family bathroom or a hotel-style ensuite, and the tiling, waterproofing and fixtures all scale up with it.
2. Scope of Work
This is the big one. There’s a world of difference between:
Layout change moving the toilet, shower or vanity means shifting plumbing and drainage, which adds real cost (and often needs council or strata sign-off).
Cosmetic refresh, new tapware, a repaint, updated mirror and lighting, maybe a new vanity. Layout and plumbing stay put.
Full renovation everything’s ripped out to the studs. New waterproofing, tiling, plumbing, electrical, and fixtures throughout.
3. Materials and Fixtures
Tiles alone can range from $30 a square metre to well over $150 for imported stone-look porcelain. Tapware, showerheads and vanities follow the same pattern a budget mixer tap might be $80, while a designer brand can run into the thousands.
4. Waterproofing and Compliance
This one’s non-negotiable, and it should never be the place you try to cut corners. Every wet area renovation in Australia must comply with AS 3740, the Australian Standard for waterproofing. Licensed waterproofing, plumbing and electrical work all add to your bathroom renovation cost, but they’re what stop you from dealing with a mould-ridden, leaking disaster two years down the track and most insurers won’t touch a claim if the work wasn’t done to code.
5. Structural Surprises
Older Australian homes especially anything built before the 1990s can hide rotten timber, ageing pipework, or even asbestos sheeting behind the tiles. It’s genuinely common for these issues to only surface once demolition starts, which is why we always recommend building in a 10–20% contingency on top of your quoted price.
6. Location
Where you live genuinely moves the needle. Labour rates, council requirements and even freight costs for materials all vary by state and city.
House Extension Cost by City
Because house extension costs genuinely differ depending on where you live labour rates, council requirements, and material freight all play a part here’s a rough regional breakdown:
House extension costs Melbourne:
Typically the priciest market in the country, often $3,000–$5,000+ per m², driven by higher labour and land costs, plus more complex council processes in areas like the Eastern Suburbs and Inner West.
House extension costs Sydney:
Sitting around the national average, roughly $2,600–$4,200 per m², with Victoria’s established building industry keeping competition (and therefore pricing) reasonably tight.
House extension cost Brisbane:
Generally a touch below Sydney and Melbourne, around $2,400–$4,000 per m², though Queensland’s stricter cyclone and wind-loading requirements in coastal areas can bump up structural costs.
House extension costs Perth:
Often the most competitive of the major cities, around $2,200–$3,800 per m², partly due to more affordable land and a strong local trades market.
House extension cost per m² Australia
For a typical mid-range bathroom renovations, here’s roughly how the budget splits:
figures always come with the caveat that your specific site, council, and design will shift these ranges a builder’s fixed-price quote after a site visit is the only truly accurate number.
Real Examples: What Different Budgets Actually Buy
$90,000–$120,000: A modest single storey extension (15–25m²) think one extra bedroom or a small living area extension, standard finishes.
$150,000–$220,000: A rear extension opening up kitchen, dining and living (30–50m²), with a mid-range finish and some structural glazing.
$250,000–$350,000: A second storey addition adding two bedrooms and a bathroom, with a new roof and staircase.
$400,000+: A full second storey plus ground floor reconfiguration, high-end finishes throughout.
Extension vs Rebuild: When Does It Stop Making Sense?
If your house extension cost estimate is creeping past 60–70% of your home’s current value, it’s worth pausing and getting a second opinion sometimes a knockdown rebuild works out better value long-term, especially on older homes needing extensive structural remediation anyway. A good builder will tell you this honestly rather than pushing you toward the bigger job.
How to Keep Your House Extension Cost Under Control
You don’t need to sacrifice quality to protect your budget. A few genuinely useful moves:
- Lock in your design before pricing every change during construction (a “variation”) costs more than deciding upfront.
- Match your extension to your existing footprint where possible extending along an existing wall line is cheaper than a complex new shape.
- Choose a builder who does a proper site inspection before quoting a fixed-price quote based on a real look at your block (not just photos) avoids nasty surprises.
- Consider a granny flat or modular addition if your goal is extra living space rather than a full integrated extension often 20–30% cheaper per square metre.
- Get 3 detailed quotes, not 3 ballpark figures ask each builder to itemise so you’re comparing apples with apples.
Getting an Accurate Quote
Every online house extension cost calculator will give you a ballpark useful for early budgeting, but not something to bank a mortgage top-up on. The only way to get a genuinely accurate figure is a site visit, a proper design brief, and a detailed, itemised quote from a licensed builder who knows your local council’s requirements inside out.
If you’re at the “just want a realistic number” stage, that’s exactly where a conversation with a local extension specialist earns its keep no obligation, just clarity on what your project will actually cost.
FAQ
1. How much does a house extension cost in Australia?
Most single storey house extensions cost between $80,000 and $250,000, or roughly $2,500–$5,000 per square metre, depending on size, finish level, and location. Second storey extensions typically cost more due to structural and roofing work.
2. How much does a second storey extension cost?
A second storey extension generally costs $3,000–$5,500 per square metre, or $200,000–$400,000+ in total, because it involves removing the existing roof, reinforcing the ground floor, and adding a staircase.
3. What is the average cost per square metre for a house extension?
The national average sits around $2,500–$3,800 per m² for a mid-range single storey extension, though high-end finishes can push this past $5,000 per m².
4. Is it cheaper to extend or knock down and rebuild?
It depends on your home’s condition and how much extra space you need. As a rule of thumb, if your extension cost is approaching 60–70% of your home’s value, a knockdown rebuild is worth comparing.
5. How much does a house extension cost per m² in Australia for a kitchen or bathroom addition?
Wet areas like kitchens and bathrooms cost more per square metre than bedrooms or living spaces typically $3,000–$4,500 per m² due to plumbing, waterproofing, and tiling costs.
6. How long does a house extension take to build?
A single storey extension typically takes 3–6 months from start to finish, while a second storey extension can take 6–12 months depending on complexity and council approval timeframes.
7. Do I need council approval for a house extension?
Most extensions require either a Development Application (DA) or Complying Development Certificate (CDC), depending on your council, zoning, and the size and location of the extension. Your builder or a town planner can confirm which pathway applies to your property.
8. How much does a house extension cost in Melbourne, Sydney, Brisbane, and Perth?
Sydney is typically the most expensive market ($3,000–$5,000+ per m²), followed by Melbourne ($2,600–$4,200 per m²) and Brisbane ($2,400–$4,000 per m²), with Perth generally the most competitive ($2,200–$3,800 per m²).
9. What’s included in a house extension cost calculator estimate?
Most online calculators give a rough per-square-metre estimate based on size and extension type, but they don’t account for site access, soil conditions, council requirements, or finish level all of which can significantly change your final price.
10. Can I finance a house extension?
Yes common options include a construction loan, refinancing to access home equity, or a dedicated renovation loan. Most lenders will want a fixed-price building contract and council approval before releasing funds, so it’s worth speaking to your builder and lender early in the planning process.