How much does a house extension cost in Australia in 2026?
A house extension in Australia costs roughly $2,000 to $4,500 per square metre in 2026, so a typical single-room ground-floor addition lands somewhere between $45,000 and $180,000, depending on size, finish and where you live. Add a second storey and you're usually looking at $3,300 to $5,500 per square metre once you factor in the extra structural work.
If you're weighing up whether to extend out the back in Brisbane, add a granny-flat-style wing in Perth, or go up with a full second storey in Sydney, the honest answer is: it depends on a handful of things that move the price a lot. This guide breaks down the real ranges, what drives the cost, and how to get a ballpark for your own place in under a minute using the free builder and renovation quote calculator.
We'll keep it plain — no jargon without an explanation, and every price anchored to what Australian builders are actually charging this year.
Last updated: July 2026.
Key takeaways
- A ground-floor house extension costs about $2,000–$4,500 per m² in 2026 — a 40 m² addition typically runs $80,000–$180,000.
- A second storey addition costs more per m² ($3,300–$5,500) because of engineering, propping and roof removal — budget $250,000–$500,000+ for a full upper floor.
- The single biggest cost driver is what's inside the walls — a new kitchen or bathroom in the extension can add $25,000–$50,000 on its own.
- The cheapest lever is keeping the roofline and plumbing simple — every wet area, skillion roof or level change adds cost.
- Council approval, engineering and site access can add 10–20% before a single brick is laid, so budget for the "before you build" bill.
What's in this guide
- House extension cost per square metre in 2026
- Ground floor vs second storey addition cost
- What drives the price of a home extension
- Approvals, engineering and hidden costs
- How to save money on a house extension
- Frequently asked questions
House extension cost per square metre in 2026
The cleanest way to budget an extension is by the square metre, then adjust for finish and complexity. Here's what the cost to extend a house looks like across the main types in 2026.
| Extension type | Typical size | Cost per m² (inc. GST) | Typical total (inc. GST) |
|---|---|---|---|
| Basic single-room ground extension | 20–30 m² | $2,000–$2,800 | $45,000–$80,000 |
| Standard ground-floor extension | 30–50 m² | $2,800–$3,800 | $85,000–$190,000 |
| High-end extension (kitchen/living, premium finish) | 40–60 m² | $3,800–$4,500+ | $170,000–$270,000 |
| Second storey addition (partial) | 40–60 m² | $3,300–$4,800 | $150,000–$290,000 |
| Full second storey addition | 80–120 m² | $3,800–$5,500 | $300,000–$600,000+ |
These ranges are based on estimates generated through Leadkit's builder and renovation calculator using current national rates, blended across metro markets. Leadkit builds the quote calculators Australian builders embed on their own sites, so the figures reflect real quote inputs rather than a guess. Sydney and inner Melbourne typically sit at the top of each range; Adelaide, Brisbane and regional areas at the lower end.
This is a price indication only. Your tradie will confirm the final price after assessing the job.
Across the extension and renovation quotes generated through Leadkit, the part homeowners most often underestimate isn't the building shell — it's the fit-out. Two extensions of the same footprint can differ by $60,000 purely on what goes inside them.
Ground floor vs second storey addition cost
A ground-floor extension is almost always cheaper per square metre than going up. Building out means new footings and a slab on relatively easy-to-access ground. A second storey addition means removing or modifying the existing roof, propping the structure, and often upgrading the footings below to carry the extra load.
Ground-floor extension. You're adding a slab, walls and a roof onto solid earth. The main variables are the slab type — a waffle pod slab (a raft of concrete poured over polystyrene void formers) is common on reactive clay sites — and whether you're cutting into a sloping block, which brings in retaining and site levelling.
Second storey addition. This is where the home extension price climbs. The existing ground floor has to be checked by a structural engineer to confirm it can carry a new level; footings may need underpinning (strengthening the foundations from below); and you'll lose the use of the house for longer. Weather protection during the roof-off stage is a real line item, not a rounding error.
As a rule of thumb, a second storey runs 20–40% more per square metre than an equivalent ground-floor addition. For a deeper look at the concrete side of a ground-floor build, our concrete slab cost per square metre guide covers slab pricing in detail.
Want a ballpark for your own extension? Use the free builder quote calculator — it takes about 30 seconds and there's no signup. Remember the result is an indication only; your builder confirms the final price on site.
What drives the price of a home extension
The biggest factor in any house extension cost in Australia isn't the number of square metres — it's what those square metres contain and how hard they are to build.
Wet areas. A new bathroom or kitchen inside the extension is the single most expensive room type per square metre. Waterproofing, plumbing rough-in, cabinetry and tiling stack up fast — a new bathroom alone commonly adds $25,000–$35,000. See our bathroom renovation cost guide for how those numbers build up.
Structural changes. Knocking out a load-bearing wall to open the old house into the new space means installing a steel beam — a universal beam (often called an RSJ, rolled steel joist) — sized by an engineer. Each opening can add $3,000–$8,000 once you include propping and making good.
Roof and rooflines. Matching a new roof to the existing one, or running a skillion roof (a single-pitch flat-ish roof) over the extension, changes both cost and council approval. Complex rooflines cost more to frame and flash.
Site access and levels. A tight side setback that a mini-excavator can't fit through, or a sloping block that needs cut-and-fill, can add 10–15% for labour and machinery alone. Builders price difficult access into every stage.
Finish level. Standard vs premium fixtures, flooring, joinery and glazing is where two identical footprints diverge by tens of thousands. The Housing Industry Association (HIA) publishes cost and activity data showing how much finish selections move the final figure.
Approvals, engineering and hidden costs
Before construction starts, there's a "soft cost" bill most homeowners forget. Budget 10–20% of the build cost for the steps that happen on paper, not on site.
- Council approval or a private certifier. Most extensions need either a Complying Development Certificate or full Development Application. Fees, plans and reports vary by council and state.
- Building designer or architect. Concept and construction drawings typically run 5–10% of the build for a designer, more for an architect.
- Structural engineering. Slab design, beam sizing and second-storey load checks are mandatory and usually $2,000–$6,000.
- Energy compliance. In NSW a BASIX certificate is required; nationally, extensions must meet the energy provisions of the National Construction Code. This can dictate glazing, insulation and orientation.
- Insurance and warranty. Home warranty insurance (called the Home Building Compensation Fund in NSW) is required on most residential work above a threshold — confirm your builder holds it.
Always check your builder's licence and insurance before signing. NSW Fair Trading (or your state equivalent — VBA in Victoria, QBCC in Queensland) lets you verify a licence in minutes, and a written, itemised contract is your best protection. The Master Builders Australia site is a solid starting point for understanding contracts and choosing a licensed builder.
A quick note on the words: a quote in Australia is legally binding, while the figures here and in any calculator are an estimate — an indicative range to help you budget. Your builder confirms the binding price after inspecting the job.
How to save money on a house extension
You can trim a home extension budget without cheapening the result, if you make the savings in the right places.
Keep the plumbing close. Locating a new bathroom or kitchen near existing plumbing stacks avoids long, expensive drainage runs.
Simplify the roofline. A single-pitch or continuation of the existing roof is cheaper to frame and flash than a complex multi-hip design.
Don't over-spec the structure. Work with the engineer's minimum compliant design rather than gold-plating footings you don't need.
Get multiple itemised quotes. Comparing three line-by-line quotes routinely saves 10–15%, and it exposes who's cutting corners. For more on comparing builder pricing, our guide on how to price trade jobs shows how builders build their numbers — useful for reading a quote critically.
Time it well. According to the Australian Bureau of Statistics (ABS), construction input costs have eased from their post-2022 peak but remain elevated, so locking a fixed-price contract protects you from mid-build rises.
Frequently asked questions
Q: How much does a house extension cost in Australia in 2026?
A: A house extension costs roughly $2,000–$4,500 per square metre in 2026 for a ground-floor addition, so a 40 m² extension typically totals $85,000–$180,000 inc. GST. A second storey addition costs more — $3,300–$5,500 per m² — because of the engineering and roof work involved. Sydney and Melbourne sit at the top of the range, while Brisbane, Adelaide and regional areas are usually cheaper. The best way to get a figure for your own place is to plug your size and finish into the builder renovation quote calculator for an instant ballpark.
Q: Is it cheaper to build up or build out?
A: Building out (a ground-floor extension) is almost always cheaper per square metre than building up (a second storey addition). Going out means a new slab on solid ground, while going up means removing the roof, propping the structure, and often strengthening the existing footings to carry the load. Expect a second storey to cost 20–40% more per square metre. Building up does have one advantage: it doesn't eat into your yard, which matters on a small block in a tight suburb.
Q: How much does a second storey addition cost?
A: A second storey addition cost in Australia typically runs $3,300–$5,500 per square metre in 2026. A partial upper floor of 40–60 m² lands around $150,000–$290,000, while a full second storey of 80–120 m² commonly reaches $300,000–$600,000 or more. The premium over a ground extension comes from structural engineering, temporary weather protection while the roof is off, and frequently upgrading the foundations. Always get a structural engineer's assessment before budgeting — some homes need significant underpinning, which changes the numbers a lot.
Q: Do I need council approval for a house extension?
A: Yes, almost all house extensions need some form of approval — either a Complying Development Certificate through a private certifier or a full Development Application through your local council. The path depends on your block size, setbacks, height and heritage status. Skipping approval risks fines and problems when you sell. Factor approval fees, plans and reports into your budget from day one, and check your state authority — such as NSW Fair Trading — for the rules that apply where you live.
Q: How long does a house extension take to build?
A: A single-room ground-floor extension usually takes 3–5 months of construction once approvals are in hand. A larger extension or a second storey addition commonly runs 5–9 months. On top of that, allow 2–4 months beforehand for design, engineering and council approval. Weather, trade availability and any variations you request during the build all stretch the timeline, so build a buffer into both your budget and your living arrangements.
Q: What adds the most cost to a home extension?
A: Wet areas add the most cost per square metre — a new kitchen or bathroom in the extension can add $25,000–$50,000 on its own because of plumbing, waterproofing, cabinetry and tiling. After that, structural changes (like removing a load-bearing wall and installing a steel beam), complex rooflines, difficult site access and premium finishes are the biggest movers. Keeping the layout simple and the plumbing close to existing services is the most effective way to keep the home extension price down.
Q: Should I renovate the existing house at the same time?
A: It often makes sense to combine an extension with renovating the adjoining rooms, because you're already paying for site setup, trades and mess. Doing a kitchen renovation as part of the extension usually costs less than doing it as a separate job later. The trade-off is a bigger single bill and longer disruption. Get the combined scope quoted both ways so you can see the real saving before you commit.
Final word: get a real number before you plan
A house extension is one of the biggest jobs a homeowner takes on, and the gap between a rough guess and a real quote is often $50,000 or more. Start with a ballpark, then get itemised quotes from at least three licensed builders and read them line by line. The ranges in this guide will tell you if a quote is in the right suburb; a proper site assessment tells you the rest.
Every price here is an estimate to help you budget. Your builder confirms the binding figure after inspecting your block, your existing structure and the finish you want.
Want an instant price estimate for your extension? Use the free builder and renovation quote calculator — it takes 30 seconds, no signup, and the result is an indication only until your tradie confirms it. Or browse all the Leadkit cost calculators to price up the other trades in your project.