A ground-floor home extension in Australia costs roughly $2,000 to $4,000 or more per square metre, so an 80 m2 extension lands between about $160,000 and $320,000 before design fees, approvals and contingency. A second storey addition starts lower per square metre for a simple build but has a much higher ceiling, and specialist builders quote complete second-storey contracts from around $350,000.
Those are the published ranges. This guide shows where each one comes from, what it includes, and why a builder's contract price is usually higher than a per-square-metre guide. All figures are Australian dollars. GST treatment is stated where the source states it; most consumer cost guides do not, so treat their figures as consumer-facing totals and confirm GST with the builder.
The short answer
| Extension type | Range | GST | Source and date |
|---|---|---|---|
| Ground-floor extension, per m2 | $2,000 to $4,000+ | Not stated | hipages brackets via Budget Direct, 29 July 2026 |
| Ground-floor extension, per m2 | $2,500 to $4,000 | Not stated | CO-architecture, 2025 guide updated 2026 |
| Ground-floor extension, 80 m2 total | $160,000 to $320,000 | Not stated | hipages via Budget Direct, 29 July 2026 |
| Second storey addition, per m2 | $1,850 simple to $3,500 typical, up to $15,000 complex | Not stated | hipages via Budget Direct, 29 July 2026 |
| Second storey addition, per m2 | $3,000 to $4,500 | Not stated | CO-architecture, 2025 guide updated 2026 |
| Second storey, 60 m2 total | $110,000 to $300,000 | Not stated | hipages via Budget Direct, 29 July 2026 |
| Ground-floor extension, builder contract total | $250,000 to $500,000+ | Included | Supa Group, Melbourne, 7 August 2026 |
| Second storey extension, builder contract total | About $350,000 to $500,000+ | Included | Supa Group, Melbourne, 10 August 2026 |
Sources: Budget Direct, How much does an extension cost, quoting hipages, 29 July 2026; CO-architecture, Home extension costs in Australia; Supa Group, How much does a home extension cost, 7 August 2026, and Cost of second storey extensions, 10 August 2026. Checked 7 October 2026.
The gap between the per-m2 guides and the Supa Group contract ranges is the most useful thing in this table. Supa Group is a Melbourne extension specialist whose contract price includes design, permits, engineering, construction and services, and excludes landscaping, furnishings and appliances. The per-m2 guides price construction alone. Both are right; they measure different things.
Ground floor or second storey?
Building out is usually cheaper per square metre than building up, provided you have the land. A ground-floor extension needs new footings and a slab but leaves the existing structure mostly alone.
A second storey needs an engineer to assess whether the existing footings and walls can carry the load, a new staircase that takes floor space from the ground level, roof removal and reinstatement, and often temporary accommodation while the house is open. That is why the hipages brackets show a simple second storey from $1,850 per m2 but a complex, high-end one up to $15,000 per m2.
Published totals for second storey additions:
| Second storey size | Range | Source |
|---|---|---|
| 60 m2 | $110,000 to $300,000 | hipages via Budget Direct, July 2026 |
| 80 m2 | $148,000 to $600,000 | hipages via Budget Direct, July 2026 |
| 100 m2 | $185,000 to $750,000 | hipages via Budget Direct, July 2026 |
| 3-bedroom addition, 90 to 120 m2 | $180,000 to $480,000 | hipages via Budget Direct, July 2026 |
CO-architecture's project totals, from its 2025 guide: a simple bedroom addition $70,000 to $120,000, a kitchen and dining extension $150,000 to $300,000, and a whole-house extension $300,000 to $600,000 or more.
A Melbourne builder's published rate
NPR Building Concepts, an eastern Melbourne extension builder, publishes a planning range of $3,500 to over $5,000 per m2 for home additions in Melbourne in 2026, depending on finish quality and site access. That sits at the top of the national per-m2 guides above, which is what a capital-city market and a specialist builder's full-service scope add. Source: NPR Building Concepts, Home addition costs, 2026.
Disclosure: Contractor Scale, which operates Bracen, provides marketing services to this builder. The figures are quoted from the builder's own published page, checked 7 October 2026, and are one company's regional examples, not a national average and not a Bracen quote.
What changes the price
- Wet areas. A kitchen or bathroom in the new space adds plumbing, waterproofing to AS 3740 and joinery. The same floor area as a bedroom can cost twice as much.
- Tie-in work. Matching roof lines, re-cladding to blend old and new, re-roofing, and upgrading old wiring or plumbing uncovered during the works. Supa Group lists these as the most common extras.
- Structure. Removing load-bearing walls to open the old house into the new space needs steel and engineering.
- Approvals and contracts. A planning permit or development application versus complying development, heritage overlays, and the design and engineering fees that go with them. CO-architecture recommends allowing 10 to 15 percent for professional fees and contingency on top of construction. In Victoria, domestic building work worth more than $10,000 must be under a major domestic building contract with a registered builder, per Consumer Affairs Victoria, updated 8 July 2025. In New South Wales, residential building work over $5,000 including GST requires a licensed contractor, per the NSW Government, updated 23 June 2026.
- Site. Access for excavation and materials, slope, soil class and tree protection.
- Living through it. A second storey often means moving out for weeks or months. Price the accommodation.
What the estimates exclude
CO-architecture's per-m2 figures exclude demolition, complex site works, utility upgrades, council fees, design fees and landscaping. Supa Group's contract figures exclude landscaping, furnishings and appliances, and it notes that variations for unforeseen conditions are additional. Add these to any benchmark before you compare it with a quote.
How to get comparable quotes
- Give each builder the same measured brief: the footprint, the rooms, the wet areas, the ceiling heights and your finish expectations.
- Ask whether the price includes design, engineering, permits and service connections, or whether you are expected to arrange them.
- Ask for a written inclusions schedule and the list of provisional sums, especially for the tie-in work where the old house meets the new.
- Ask how unforeseen conditions in the existing structure will be assessed and priced.
- If the extension is approaching new-build money, read our knock down rebuild cost guide before you commit.
For a one or two bedroom detached unit in the backyard, a granny flat is a different approvals path and often a different price.
The estimator below this article gives a modelled starting range from your answers, in Australian dollars. It does not inspect your home or replace a priced scope.
Frequently Asked Questions
How much is a ground-floor extension per square metre?
About $2,000 to $4,000 or more per m2 according to hipages brackets published by Budget Direct in July 2026, or $2,500 to $4,000 per m2 according to CO-architecture. An 80 m2 extension typically lands between $160,000 and $320,000 for construction alone.
How much is a second storey addition?
From roughly $1,850 to $3,500 per m2 for a simple build, rising to around $15,000 per m2 for large, complex, high-end work (hipages via Budget Direct, July 2026). CO-architecture quotes $3,000 to $4,500 per m2. A 60 m2 second storey typically costs $110,000 to $300,000.
Why do builder quotes come in higher than per-square-metre guides?
Specialist builders price the whole project. Supa Group's Melbourne contract ranges of $250,000 to $500,000 or more for a ground-floor extension and from about $350,000 for a second storey include design, permits, engineering and services that per-m2 guides leave out, and include GST.
Does the price include GST?
Supa Group states its ranges include GST. Budget Direct and CO-architecture do not state a GST position. Ask every builder for a written total that states whether GST is included, and compare quotes on the same basis.
How much contingency should I hold for an extension?
At least 10 to 15 percent on top of the construction budget, which is what CO-architecture recommends for professional fees and contingency combined. Older homes with unknown wiring, plumbing or framing conditions justify the higher end.