House extension cost in Australia
Ground floor extensions run $2,800–$7,000 per m²; a second storey costs 30 to 50% more again. Here's what that premium actually pays for, and the costs that never appear in a per-square-metre rate.
What an extension costs in Australia
Extensions are priced per square metre of new floor area, and the rate depends most on whether you're building out or building up.
- Ground floor extension, standard finish — roughly $2,800 to $4,500 per m²
- Ground floor extension, high-end finish — roughly $4,500 to $7,000 per m²
- Second storey addition, standard build — roughly $3,800 to $5,500 per m²
In project terms: a 40m² family room addition commonly runs $100,000 to $180,000. An 80m² ground floor extension at standard quality lands around $224,000 to $360,000 in construction cost. A full second storey typically sits somewhere between $250,000 and $600,000 depending on size, location and the condition of what's underneath it.
Sydney and Melbourne sit at the top of their state ranges. Regional pricing is generally lower on labour, though access and material freight can claw some of that back.
Two identical 40m² additions on neighbouring blocks can differ by tens of thousands, because the cost is driven as much by the house you already have as by the space you're adding. Every per-square-metre figure on this page is a planning estimate. The only number that means anything is a site-specific fixed-price quote.
Building up costs 30 to 50% more, and here's why
A second storey typically runs 30 to 50% more per square metre than an equivalent ground floor extension. That premium isn't margin — it's a list of things a ground floor addition simply doesn't have to do.
The existing structure has to carry it. The ground floor was engineered for a roof, not for another storey. Footings, walls and framing frequently need reinforcement before anything goes on top, and that work happens inside a house you're living in.
The roof comes off. Removal and full reconstruction, with the house exposed in between. That drives weather protection, sequencing and risk.
A staircase has to go somewhere. It consumes ground floor area you already had, so you lose space downstairs to gain it upstairs, and the reconfiguration around it is its own job.
Height changes the rules. Scaffolding, fall protection and height safety requirements extend timelines and add cost across every trade on site.
What building up buys you is your block. You keep the garden, the outdoor living area and the ground floor's daylight. On a small or tight site, that's frequently worth the premium — and on a large block, it frequently isn't.
The costs that aren't in the per-square-metre rate
Published rates generally cover construction: builder's margin, preliminaries, GST. They generally exclude a substantial list of things you will still pay for.
Design and documentation. Architect or building designer, structural engineering, energy compliance assessment.
Approval. In NSW the pathway is either a Complying Development Certificate through a private certifier, typically two to six weeks, or a full Development Application through council, typically six to sixteen weeks. Which one you're eligible for depends on your site and your design, and it materially affects both cost and timeline.
Make-good to the existing house. The new part meets the old part somewhere, and that junction is rarely tidy. Floor levels don't match, ceiling heights differ, the existing roof has to be tied into the new one, and finishes have to be blended or replaced across the boundary.
What's found once walls open. Older homes routinely surface asbestos, non-compliant wiring, rot or termite damage. In a home built before the 1990s, budget for the possibility of asbestos identification and licensed removal.
Living somewhere else. Second storey work in particular can make a house uninhabitable for a period. Temporary accommodation is a real line item that appears on no builder's quote.
Adding a bathroom, ensuite or kitchen to an extension raises the rate for that portion of the floor area considerably, because of plumbing, waterproofing and ventilation. A bedroom-and-living extension and the same area including a bathroom are different projects at different prices.
Extend, or knock down and rebuild?
The rough industry rule: extending is cheaper for anything under about 50m². Beyond that the gap narrows quickly, because a large extension increasingly carries the costs of a new build while still inheriting the constraints of the old house.
The case for extending is that you keep what already works, you avoid demolition and disposal, and you may keep aspects of the existing home that current planning controls wouldn't let you rebuild.
The case for rebuilding is that you stop paying to work around a structure that was never designed for what you're now asking of it — and you get a house built to current standards throughout, rather than a new wing attached to an old one.
Worth pricing both properly if your extension is heading past 50m². The answer is site-specific and it surprises people in both directions.
Can your house actually carry a second storey?
Everything about building up assumes the house underneath can take it. That assumption is worth testing before you spend money on design, because the answer changes the budget by tens of thousands and occasionally ends the project.
Adding a storey roughly doubles the load on your existing footings, which on most Australian homes were sized for a single storey and nothing more. Homes built before the 1980s were generally designed as single-storey structures outright. Newer slab-on-ground homes with brick veneer and timber trusses were also engineered for single-level loads, so age alone doesn’t settle it.
What the assessment involves
A structural engineer starts with a visual inspection: cracking in masonry, signs of slab movement, distortion in door frames. If the original house plans exist, they give footing sizes, beam depths and the soil classification. If they don’t, a geotechnical test is ordered — a rig drills several metres and the samples go to a laboratory.
What comes back is a soil class under AS 2870, ranging from Class S, slightly reactive, through M and H to Class E, extremely reactive. The higher the reactivity, the more the ground moves through wet and dry cycles, and the more stress that puts on footings that are about to carry twice what they were designed for.
That last line is the one that moves projects. It is not a certainty — plenty of homes need no strengthening at all — but it is a realistic possibility on an older home or a reactive site, and it is entirely invisible until someone assesses it.
The structural assessment is the definitive answer for your specific house, and it should be one of the first things you pay for rather than one of the last. Commissioning a design first and testing feasibility second means you may pay to design something the house can’t carry, then pay again to redesign it. A few thousand dollars spent early is the cheapest insurance available on a project this size.
What the engineer might specify
Where strengthening is needed, the common measures are underpinning or otherwise strengthening existing footings, steel beams to redistribute load, reinforcing or replacing wall framing, and new structural columns. Full brick construction, brick veneer and timber frame all behave differently, and homes on pier-and-beam footings generally need less work than slab-on-ground.
None of this appears in a per-square-metre rate. It is site-specific engineering, and two identical designs on two neighbouring blocks can carry very different structural costs.
Frequently asked questions
Can any house have a second storey added?
Most can, with appropriate engineering, but the cost of getting there varies enormously. Adding a storey roughly doubles the load on existing footings, which on most Australian homes were sized for a single storey. Homes built before the 1980s were generally designed as single-storey structures outright, and newer slab-on-ground homes were also engineered for single-level loads. A structural engineer's assessment is the definitive answer for your specific house.
What does a structural assessment cost before a second storey?
A geotechnical soil report runs $500 to $2,000 depending on the number of bores and depth of analysis, and structural engineering for assessment and certification runs $2,000 to $8,000. Where the footings need strengthening, underpinning commonly runs $15,000 to $35,000. Commission the engineer before the architect — designing first and testing feasibility second can mean paying to design something the house can't carry.
What is a soil classification and why does it matter?
Under AS 2870, soils are classified from Class S (slightly reactive) through M and H to Class E (extremely reactive). The classification describes how much the ground moves through wet and dry cycles. More reactive soil means more movement and more stress on footings that are about to carry twice their design load, so it directly drives what strengthening your project needs. If the original house plans exist they may state it; otherwise a geotechnical test establishes it.
How much does a house extension cost in Australia?
Ground floor extensions run roughly $2,800 to $4,500 per m² at standard finish and $4,500 to $7,000 for high-end. A 40m² addition commonly comes to $100,000 to $180,000, and an 80m² extension $224,000 to $360,000 in construction cost.
Is a second storey more expensive than extending out?
Typically 30 to 50% more per square metre. The premium covers structural reinforcement of the existing ground floor, removing and rebuilding the roof, a staircase that consumes downstairs floor area, and the height safety requirements that affect every trade on site. What it buys you is your backyard.
Do I need council approval to extend?
In almost all cases. In NSW the pathway is either a Complying Development Certificate through a private certifier, usually two to six weeks, or a Development Application through council, usually six to sixteen weeks. Which applies depends on your site and design.
What's not included in a per-square-metre rate?
Usually design and documentation, engineering, council and certifier fees, landscaping, make-good where new meets old, anything discovered once walls are opened, and temporary accommodation if you can't live there during the work. Published rates generally cover construction only.
Is it cheaper to extend or knock down and rebuild?
Extending is generally cheaper under about 50m². Past that the gap closes fast, because a large extension carries much of the cost of a new build while still working around the existing structure. If you're heading past 50m², price both.
Sources & further reading
- Structural assessment, geotechnical report and underpinning cost ranges cross-checked against 2026 published Australian pricing from structural engineers, building consultants and second-storey specialists across Sydney and Melbourne.
- Soil classifications (Class S through Class E) are defined by AS 2870, Residential slabs and footings. Residential construction standards are set by the National Construction Code Volume Two, with site-specific requirements added by local Development Control Plans and Local Environmental Plans.
- Australian per-square-metre extension pricing cross-checked against 2026 published data from Australian builders, architects and industry sources, including Housing Industry Association activity data, CO-architecture and Australian extension builders.
- Approval pathways and timeframes described are for NSW. Requirements and terminology differ in other states and territories.
- For a real itemised project estimate on a second dwelling, including service connections and subsidence footing costs, see our dual occupancy cost guide.
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Cost ranges here are planning estimates that vary by region and home — see how we research and what our numbers mean, and always confirm with local, itemised quotes.