House Extension Cost 2026: A Realistic Guide to Building Costs
Summary
The short version. A single-storey rear extension with standard finishes runs $2,800 to $4,200 per square metre in 2026. Add a kitchen or bathroom and that climbs to $4,200 to $6,000 per square metre. A second-storey addition sits between $4,500 and $7,000 per square metre because you are paying for structural upgrades to a house that was never designed to carry a floor above it. On top of the build, budget $12,000 to $25,000 for drawings, engineering, reports, and approvals before a single brick moves. The square metre rate in a brochure is not the number you will pay, and the gap between the two is where most owner-builder budgets come apart.
Table of Contents
A $121,000 difference between two quotes for the same extension is enough to make anyone question what is going on.
The project itself was straightforward on paper: a 38 square metre single-storey rear extension with a new living and dining area, slab on ground, and one bathroom. One builder came back at $147,000. Another, pricing from the same hand sketch only three weeks later, came back at $268,000.
The problem wasn’t that one builder was trying to take advantage of the client. They were pricing two very different versions of the same vague idea.
The lower quote covered a basic shell with a plasterboard ceiling, builder’s-range tapware, and no allowance for the retaining wall the sloping block clearly required. The higher quote included excavation, a new switchboard, and a roof connection that properly tied the extension into the existing hip roof. Once we documented the project properly and both builders priced the same scope, the difference between them dropped to less than $19,000.
That gap says a lot about house extension costs in Australia. Large differences between quotes often come down to what has, and hasn’t, been included. Until the extension is properly documented, builders are filling in the blanks themselves.
The figures below are based on the extension projects we see across New South Wales, Victoria, Queensland, and Tasmania, including the design fees, approvals, site costs, service upgrades, and other expenses that often never make it into the first budget.
What an extension actually costs in 2026
Our working figures, based on what our clients are being quoted through 2025 and into 2026, look like this. A simple single-storey addition with a concrete slab, standard aluminium windows, plasterboard lining, and no wet areas comes in at $2,800 to $4,200 per square metre of new floor area. Put a kitchen, laundry, or bathroom inside it and you are at $4,200 to $6,000 per square metre, because wet areas carry waterproofing, tiling, cabinetry, and plumbing that cost three to four times what an empty room costs. A second-storey addition runs $4,500 to $7,000 per square metre.
Sloping sites, tight access with no room for a truck or pump, heritage overlays, and bushfire construction requirements each add 10 to 30 per cent. Those are not rare conditions. In older inner suburbs they are closer to the default.
The whole job, stage by stage, with dollars attached
The number that matters is not the per-square-metre rate. It is the total, sequenced, with dates attached, because an owner-builder is paying for time as well as materials. The table below is a 40 square metre single-storey rear extension with one bathroom, on a reasonably flat block with side access. Total construction $175,000 to $240,000, soft costs $14,000 to $26,000 on top.
| Stage | Realistic duration | Cost | What trips people up |
|---|---|---|---|
| Concept design and measure-up (feature/internal survey) | 1–3 weeks | $1,500–$4,000 | Existing house is never square. Measured survey saves rework later. |
| Working drawings, engineering, reports | 3–6 weeks | $4,500–$12,000 | Consultant reports run in parallel, not sequence, if you book them early. |
| Approval (CDC, DA, or building permit) | 3 weeks – 7 months | $1,500–$6,500 | A DA with neighbour objections is where six months disappears. |
| Owner-builder permit and insurance | 2–6 weeks | $400–$2,200 | Cannot be applied for after work starts. |
| Pricing, trade bookings, contracts | 3–6 weeks | $0–$2,500 | Good trades are booked 8–14 weeks out. |
| Demolition, excavation, slab or subfloor | 2–5 weeks | 12–18% of build | Rock, old footings, and drainage you did not know existed. |
| Frame, roof, windows to lock-up | 4–8 weeks | 28–35% of build | Window lead times of 6–10 weeks if ordered late. |
| Services rough-in, insulation, lining | 3–6 weeks | 15–20% of build | Switchboard and stormwater upgrades. |
| Fit-out, tiling, joinery, painting | 6–12 weeks | 30–38% of build | Where scope creep eats the contingency. |
| Final inspections and occupancy | 1–4 weeks | $500–$1,500 | Inspections are booked, not summoned. |
Where the money goes inside each stage
Stages are easy to list and hard to fund, so here is what each one buys and where the surprises sit. Percentages are of construction cost, not the all-in project cost.
Design and documentation: $6,000 to $16,000
This covers the measure-up, the concept, the working drawings, the structural engineering, the energy assessment, and whatever site reports your council or certifier requires. Structural engineering for a straightforward single-storey addition is $2,000 to $4,500. A second storey pushes that to $3,500 to $7,000 because the engineer has to assess existing footings and specify how load gets to the ground. Soil classification is $600 to $1,200. An energy assessment (BASIX in New South Wales, a NatHERS rating in Victoria, Queensland, and Tasmania) is $250 to $600. A bushfire attack level report, if your property is mapped, is $700 to $2,000.
One thing we strongly recommend is never getting a construction price from a set of preliminary plans. Preliminary drawings are there to establish the design, not tell a builder exactly how the project is to be constructed. There are too many gaps around structure, junctions, materials, services, waterproofing, finishes, and site conditions. When those details are missing, the builder has to make assumptions, and those assumptions are normally priced with enough margin to protect them if the job turns out to be more difficult than expected.
That is why skipping proper documentation, what we call a construction set, to save $4,000 can be one of the most expensive decisions an owner-builder makes. A detailed construction set gives each builder the same scope to price. Without it, you are not comparing quotes for the same job. You are comparing each builder’s interpretation of what they think the job might involve, and the uncertainty either gets marked up at quotation stage or comes back later as a variation.
Approvals: $1,500 to $6,500
What you pay depends on the pathway, not the size. In New South Wales, a compliant single-storey addition can sometimes go through as a Complying Development Certificate for $1,500 to $3,500 and four to six weeks. The same job that triggers a Development Application because of a setback, a floor space ratio, or a heritage listing costs $2,500 to $6,500 in council fees and takes three to seven months. Victoria splits planning permit and building permit, and a building permit for an extension of this size is typically $2,200 to $3,800 with a private surveyor. Queensland building approval through a private certifier runs $1,800 to $3,500, with additional cost if a siting variation is needed. Tasmania’s pathway depends on the permit category your work falls into, and the fee difference between a notifiable job and a permitted one is meaningful.
Owner-builder permits: $200 to $2,200
Each state sets a value threshold above which an owner-builder needs a permit or certificate of consent, and every extension we are talking about here is well over it. Thresholds as at early 2026 sit at $10,000 in New South Wales, $16,000 in Victoria, and $11,000 in Queensland. Tasmania works through Consumer, Building and Occupational Services and the categories are structured differently, so check the current guidance rather than assuming. Victoria’s certificate of consent is the most expensive of the four at roughly $1,500, plus the online knowledge assessment. New South Wales and Queensland are cheaper but require a white card and a course. None of these can be obtained retrospectively, which we have watched cost a client eight weeks of dead time.
Structure and lock-up: 40 to 53 per cent of build cost
This is the heavy end. Demolition and excavation on our 40 square metre example is $8,000 to $22,000, and the top of that range is where rock, an old outhouse footing, or a stormwater line under the new slab shows up. Slab and footings are $18,000 to $34,000. Frame, roof structure, roofing, fascia, and gutter come in at $32,000 to $52,000. Windows and external doors are $9,000 to $28,000 depending on whether you are using standard aluminium or a thermally broken commercial section. Brick or cladding to match the existing house is $14,000 to $30,000, and matching a 1950s brick is a genuine cost item, not a rounding error.
Services, lining, and fit-out: 47 to 58 per cent of build cost
Plumbing and drainage for the addition and new bathroom is $12,000 to $24,000. Electrical is $7,000 to $16,000, plus $2,500 to $6,000 if the existing switchboard needs replacing, which on a house with ceramic fuses it will. Insulation and plasterboard is $9,000 to $17,000. The bathroom, fully finished, is $18,000 to $38,000. Kitchen joinery, if the extension includes one, is $14,000 to $45,000. Painting is $6,000 to $13,000, flooring $5,000 to $18,000.
Contingency: 10 to 15 per cent, held in cash
Not a line item. A rule. On a renovation you are cutting into a building somebody else built, often without drawings, and the odds of finding nothing unexpected are close to zero. On a $200,000 extension that is $20,000 to $30,000 sitting in the account and not spent on better tapware in week three.
Owner-builder checklist. We built a free tool to help owner-builders plan and organise the different parts of a project before work starts. It gives you a structured place to work through the items you need to allow for, track what has been organised, and see what still needs attention. Use the owner-builder checklist to get your project organised before you commit to the build.
The costs that never appear in a square metre rate
Every second extension website in the country advertises a rate somewhere between $1,800 and $2,400 per square metre. It is not a lie exactly. It is a shell price for new floor area on a flat, clear site, and it excludes demolition, excavation, tie-in to the existing roof and walls, service upgrades, floor coverings, wet areas, painting, and approvals. The rate is real, the job it describes does not exist.
The closest everyday comparison is a supermarket shelf tag that covers the packet but charges you separately for the trolley, the bag, and the walk to the car park. You still leave with groceries. You just leave having paid 60 per cent more than the tag suggested, and the tag was never going to tell you that.
Then there are the costs that sit outside the build entirely. If the extension takes your kitchen out for fourteen weeks, that is $2,000 to $6,000 in eating out unless somebody plans a temporary kitchen. If a second-storey addition means moving out for four months, rent is $9,000 to $28,000 depending on where you live. Owner-builder construction insurance and public liability is $1,200 to $3,500. Temporary fencing and a site toilet is $1,500 to $4,000. Skip bins, and you will need four to eight of them, are $450 to $900 each. Scaffolding for a second storey is $4,000 to $12,000 for the hire period, and the hire period always outlasts the estimate.
What Draftee does. We are an architectural drafting firm. We draw the extension so it can be approved and priced, coordinate the engineer and the energy assessor, and hand you a documented set that a builder or your own trades can quote line by line. We draw the plans, we don’t sell buildings. That matters when you are reading cost advice, because a company quoting you a square metre rate has a product to move and a margin to protect. We do not have a single reason to steer you toward a bigger extension than you need.
How owner-builders take real money out of the number
The savings that actually work on an extension are geometric, not cosmetic. Keeping the addition inside a simple rectangle with a single roof pitch saves more than any tapware decision you will ever make, because every external corner, every roof valley, and every change in ceiling height multiplies labour. We have redrawn extensions where deleting one bay window and squaring off a stepped rear wall pulled $22,000 out of the build without losing a square metre of floor.
Put the new wet areas against the existing ones. A bathroom that backs onto the current plumbing stack saves $4,000 to $9,000 in drainage. Keep the floor level consistent with the existing house rather than stepping down to follow the fall, unless the fall is severe, because a stepped slab brings retaining and waterproofing with it.
Order windows the week approval lands. Six to ten week lead times are normal, and a frame standing open for three weeks waiting on glazing is three weeks of your trades being somewhere else.
The last one is the least popular. Build smaller and finish it properly. A 32 square metre extension with decent joinery, good glazing, and a ceiling height that works will outperform a 48 square metre extension finished on a stretched budget, and it will cost $60,000 less. Most of the regret we hear is about quality, not area.
Ready to draw? If you have a rough idea of what you want and you need it documented properly so it can be approved and priced without guesswork, that is the part we do. Send us the block, the existing house, and what you are trying to achieve, and we will tell you what the drawing package involves and what it costs. No sales call, no package deal, no product to sell you. → Tell us about your block
Questions we get asked about extension costs
Per square metre, a detached granny flat is usually cheaper than an extension, because a new building on clear ground has no demolition, no tie-in, and no working around an occupied house. Our clients typically see $2,400 to $3,600 per square metre for a detached two-bedroom flat against $2,800 to $6,000 for attached extension floor area. If your goal is a self-contained space for family or rent rather than a bigger living area, read our Australian Granny Flat Guide before you commit to extending. A knock-down rebuild only makes sense when the existing house is structurally tired, because you are paying $18,000 to $35,000 in demolition before you gain anything.
Expect 45 to 70 per cent more per square metre. The cost is not the new floor. It is what sits under it. Existing footings often need underpinning or new pad footings, the ground-floor walls may need bracing or replacement lintels, you need a stair (which eats 6 to 9 square metres of ground-floor space you already use), and you need scaffolding for the whole job. Add the roof, which has to come off and go back on. On a 60 square metre second storey we see $270,000 to $420,000 for construction, plus four months of rent if the house becomes unliveable. A ground-floor addition of the same area lands $170,000 to $310,000 and you usually stay in the house.
You can save on labour you are genuinely capable of doing, which for most owner-builders means demolition, site clean, insulation, painting, landscaping, and some carpentry. Realistically that is 8 to 15 per cent of the build, not 30. Where owner-builders actually save is in managing the job and paying trades directly instead of paying a builder’s margin and preliminaries, which is typically 18 to 28 per cent of contract value. That saving is real, and it is earned with about 10 to 15 hours a week of your own time for six to nine months. Plumbing, electrical, waterproofing, and gas must go to licensed trades regardless.
From first sketch to final inspection, ten to eighteen months is normal for a single-storey extension where the approval runs smoothly. A Development Application with objections, or a heritage referral, adds three to six months to the front of that. Our stage-by-stage breakdown of a comparable project, How Long Does It Take To Build a Granny Flat? Stage-by-Stage Timeline, maps the same sequencing logic and the same places where owner-builders lose weeks. The pattern holds for extensions, with demolition and tie-in adding two to four weeks at the start.
Yes, and the driver is labour availability more than materials. Sydney and Melbourne metro sit at the top of every range in this article. Brisbane is 5 to 12 per cent below Sydney on comparable work, and Hobart and Launceston vary wildly by trade availability, where a single specialist trade being booked out for eight weeks changes your program more than any price difference. Regional areas can run higher than metro once you add travel time for trades. For a sense of how much one city diverges from another on comparable new work, our figures for Ggranny Flat Cost Melbourne and Granny Flat Cost Perth show the spread clearly.
Service upgrades. Nobody budgets for them because they are invisible until a trade opens a wall. An extension commonly triggers a new switchboard ($2,500 to $6,000), a stormwater redesign to handle the extra roof area ($3,000 to $9,000), a sewer connection amendment or an inspection of a line running under the new slab ($2,000 to $7,000), and sometimes a water meter or main upgrade ($1,500 to $4,500). On a 40 square metre addition that is potentially $9,000 to $26,000 of work that appears in month two and was in nobody’s spreadsheet in month zero. We flag what we can see on the drawings. The rest is why you hold contingency.
Continue Your Research
Knock Down Rebuild Cost 2026: What the Whole Job Actually Costs
Sloping Block House Design: Cost, Approvals, and What the Fall Decides
The Importance of Professional Land Surveys: Why the Fence Isn’t the Boundary
Last updated and changelog
Last updated: 17 September 2026
Changelog:
– 17 September 2026: Initial publication
