House Extension Cost Australia: 2026 Builder's Guide

Posted on May 13, 2026
Calum
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TL;DR: In 2026 a house extension in Australia typically runs $2,800 to $5,500 per square metre, against $2,400 to $3,800/m2 for a new build. A 30m2 rear kitchen and dining addition lands near $162,500 once permits, demolition and site costs are in. A 50m2 master suite is closer to $248,000. An 80m2 second storey can hit $470,000. Most homeowners budget 30 to 40 per cent short because they price the build and forget the footings, the services, the asbestos and the rent while they are out of the house.

Extensions are the most misquoted job in Australian residential construction. A homeowner gets a $200,000 figure from a mate at the pub, then a $480,000 quote from a registered builder, and nobody explains the gap. That gap is the whole subject of this guide. The honest version of house extension cost Australia in 2026 is that you are not buying square metres, you are buying complex renovation work bolted onto a structure that was built to a different code, by someone who is now legally responsible for both halves.

What follows is per square metre pricing by extension type, three worked examples with the line items shown, the costs that reliably blow budgets, how the numbers move state by state, what the National Construction Code adds, and how to read three quotes without getting burned. Treat every figure here as an indicative range from typical metropolitan projects. It is a starting point for a conversation with a builder, not a quote.

Why two quotes for the same extension can be $200,000 apart

The pub number is a build cost. The builder's number is a build cost plus preliminaries, plus supervision, plus insurances, plus contingency for what is behind the wall, plus margin.

That last item is where most of the argument happens, and it is worth being straight about it. A registered builder carrying home warranty insurance, public liability, a supervisor's time and a fixed-price contract is not charging the same as a bloke doing cash weekends. Margin on residential work in Australia usually sits well clear of what homeowners assume, and if you want the actual arithmetic on how builders price overheads versus profit, we broke it down in our guide to construction markup percentage in Australia.

The second reason for the gap is scope. The cheap quote almost always excludes the things that are hardest to price and easiest to leave off a page: footings to suit the new load, sewer relocation, asbestos, temporary works, making good the driveway. None of those are optional. They just move from the quote to a variation later, which is the worst place for them to live.

Extension or knockdown rebuild: run both numbers first

Before you commit to extending, do the maths on a knockdown rebuild. In 2026 the extension premium per square metre is often higher than building new, because you are working around what is already there.

New builds sit around $2,400 to $3,800/m2 in most capital cities. Extensions run $2,800 to $5,500/m2 depending on type.

Extending makes sense when:

The existing house is structurally sound and reasonably modern, roughly post-2000
You like the suburb and the land value justifies the spend
You need 30 to 80m2 more space, not a full reimagining
A heritage overlay or character zone stops you demolishing anyway

A knockdown rebuild usually wins when the existing house has asbestos through it, failing stumps, original wiring, or when you need more than about 100m2 of extra space. The demolition itself is typically $25,000 to $45,000 including disposal, and you start on a clean site that is far cheaper to build on.

House extension cost Australia: rates per square metre by type

These rates assume standard finishes, a metropolitan location and reasonable site access. Premium finishes add 25 to 40 per cent on top. Tight access, a battleaxe block or a crane over a neighbour's roof will add more again.

Extension type	Cost per m2 (2026)	Typical project range	Build time
Ground floor extension	$2,800 to $4,500	$85k to $360k	4 to 7 months
Second storey addition	$3,200 to $5,500	$160k to $440k	6 to 10 months
Granny flat (turnkey)	$2,200 to $4,200	$130k to $250k	3 to 5 months
Garage conversion	$1,800 to $3,200	$45k to $95k	2 to 4 months
Wraparound extension	$3,000 to $4,800	$180k to $480k	5 to 9 months
Knockdown rebuild	$2,400 to $3,800	$480k to $950k	9 to 14 months

Why second storeys cost more: engineering. You are loading existing footings that were never designed for it, which usually means pier reinforcement, structural steel transfer beams, temporary roof removal and crane hire. Add scaffolding for four to six months and the per m2 figure climbs fast.

Why granny flats look cheap per m2: they are standalone, single storey, simple roof lines, and most come off pre-engineered designs. The catch is that site costs and service connections can add $30,000 to $60,000 if the block is not easy to service.

Three worked examples with the line items shown

30m2 kitchen and dining extension

Standard rear addition opening into the existing living area. Single storey, brick veneer to match, Colorbond roof, mid-range kitchen.

Build: 30m2 x $3,400/m2 = $102,000
Kitchen joinery and appliances: $35,000
Demolition of existing rear wall: $4,500
Council DA, plans and engineering: $9,000
Site costs, services and drainage: $12,000
Total: $162,500

50m2 master suite, ground floor

Bedroom, walk-in robe, ensuite with double vanity, study nook. Mid to high finish.

Build: 50m2 x $3,800/m2 = $190,000
Ensuite fixtures and tiling premium: $18,000
Joinery for the WIR and study: $14,000
Permits and design: $11,000
Site and services: $15,000
Total: $248,000

80m2 second storey addition

Three bedrooms, family bathroom, void over the stairs. Existing single storey weatherboard.

Build: 80m2 x $4,400/m2 = $352,000
Footing reinforcement and structural steel: $38,000
Stairs and void: $22,000
Roof removal, crane and scaffolding: $26,000
Permits, engineering and energy reports: $14,000
Owner accommodation, 12 weeks: $18,000
Total: $470,000

Notice what the build cost is as a share of each total. On the 30m2 job it is 63 per cent. On the second storey it is 75 per cent. Everything else is the stuff people leave out of the plan and then find in the contract.

The line items that blow extension budgets

The headline per m2 rate is the easy bit. These are the items that actually move a job into overrun:

Foundation surprises: $8,000 to $25,000. Underpinning, replacing failed stumps, or upgrading footings to carry the new load. Reactive clay sites in Adelaide and parts of Melbourne are notorious, and your soil classification under AS 2870 (class A through to class P) drives the entire footing design.
Services rerouting: $5,000 to $18,000. Sewer, stormwater, gas mains and electrical. NBN and Telstra pit relocations alone can run $4,000 to $8,000.
Asbestos removal: $3,500 to $15,000 depending on quantity. Safe Work Australia notes that any home built or renovated before 1990 is likely to contain asbestos, and in an extension it turns up in eaves, cladding, vinyl tiles and fence sheets.
Council fees and contributions: $4,000 to $22,000. DA or CDC lodgement, Section 7.11 contributions in NSW, open space levies, BASIX and NatHERS reports.
Structural matching: $3,000 to $12,000. Matching old brick, render texture or roof tiles often means a custom blend or an import, and sometimes you cannot match it at all.
Temporary accommodation: $1,500 to $3,500 a week if you cannot live through it, for three to six months.
Driveway and landscape repair: $5,000 to $15,000 to fix what the trucks, the scaffold and the crews chew up.

What the NCC and AS 1684 add to the bill

An extension is new work, so it is built to the current code even though the house beside it was not. That is the part homeowners find hardest to accept and the part builders price without blinking.

Timber framing is designed to AS 1684, which sets member sizes against wind classification, span and spacing. A single storey addition in a sheltered suburb and the same addition on an exposed corner block are different framing packages. In cyclonic wind regions in far north Queensland the tie-down and bracing requirements alone add roughly 8 to 12 per cent, and termite management is mandatory in Queensland.

Stairs are the second storey item people underprice. Under the National Construction Code, private stair risers must fall between 115mm and 190mm, goings between 240mm and 355mm, and 2R plus G between 550mm and 700mm. Balustrades are required wherever the fall exceeds 1m, at 865mm minimum above the stair nosing line and 1m above a landing, with no gap allowing a 125mm sphere through. Get the rise and go wrong at design stage and you lose floor area upstairs fixing it. Our stair calculator guide walks the arithmetic.

Codes change, and the edition adopted differs by state. Check the current provisions for your jurisdiction on the ABCB's NCC site rather than trusting a set of plans drawn three years ago.

State by state variation in 2026

Geography matters more than people think. The same 50m2 extension can swing $80,000 between states.

NSW (Sydney metro): highest, at $3,400 to $5,500/m2. DA timelines and Section 7.11 contributions add $8,000 to $25,000. Regional NSW drops to $2,800 to $4,000/m2.
Victoria (Melbourne): $3,200 to $4,800/m2. Reactive clay drives footing costs up, and heritage overlays through the inner suburbs add design fees and time.
Queensland (Brisbane and Gold Coast): $2,800 to $4,200/m2. Cyclone tie-down north of Bundaberg adds 8 to 12 per cent. Termite barriers are mandatory.
WA (Perth): $2,900 to $4,300/m2. Sandy soils make footings cheaper, but trade availability and freight on materials work the other way.
SA (Adelaide): generally the most affordable capital for the build itself, though highly reactive clay through the eastern and northern suburbs pushes footing and underpinning costs toward the top of the $8,000 to $25,000 range.
Tasmania, ACT and NT: thinner subcontractor pools and freight costs mean quotes vary far more between builders than they do in the eastern capitals. Get three, not two.

GST, and the owner-builder question

Residential building work carries 10 per cent GST, and a builder's quote to a homeowner should be GST inclusive. If a number looks 10 per cent better than the others, check that first before you check anything else. A quote showing "$240,000 plus GST" is a $264,000 job.

Owner-builders do not escape GST on materials, and unless you are registered for GST and building for a business purpose, you cannot claim it back. What owner-building saves is the builder's margin and supervision, and what it costs you is time, risk and the legal responsibility for the work. Every state sets a dollar threshold above which you need a permit, plus its own insurance and defect liability rules. We covered the thresholds and the paperwork state by state in the Australian owner-builder permits guide.

How to compare three extension quotes without getting burned

Three quotes are useless if they are pricing three different jobs. Before you compare a single dollar figure, line them up on these:

Is the scope identical? Same demolition, same site works, same fixture allowances, same external finishes.
What are the provisional sums and prime cost items? A $6,000 PC allowance for a kitchen and a $30,000 one produce very different totals from the same builder.
Are site costs included or excluded? This is the single most common exclusion, and it is the biggest one.
Is there a contingency, and whose is it? On an extension, 10 to 15 per cent is realistic given what is behind the walls.
What is the payment schedule against the stages? Deposit caps and progress payment stages are regulated in most states.
Who holds the risk for asbestos and footings? If the answer is "we will deal with it if we find it", that is a variation waiting to be signed.

If you are the one issuing the quote rather than reading it, our construction quote template guide has the inclusions and exclusions layout that stops these arguments before they start.

Frequently asked questions

How much does a 40m2 extension cost in Australia?
On the ground floor at $2,800 to $4,500/m2, the build alone is roughly $112,000 to $180,000. Add permits, site costs and finishes and a realistic all-in figure is $150,000 to $230,000 depending on state and finish level.

Is a second storey cheaper than a ground floor extension?
Almost never per square metre. You avoid buying land and you keep your yard, but you pay for footing reinforcement, transfer beams, temporary roofing, crane and scaffolding. It wins on small blocks where there is nowhere else to go.

How long does a house extension take?
Four to seven months on the ground floor, six to ten months for a second storey, plus design and approvals before any of it starts. Council approval alone can run two to six months depending on the pathway and the overlay.

Do I need council approval for an extension?
Almost always. Some minor works fall under exempt or complying development in some states, but anything adding habitable floor area or changing the roof line generally needs a full application. Confirm the pathway with your local council before you pay for detailed drawings.

How much should I keep as contingency?
Ten to 15 per cent of the contract sum for an extension, and closer to 15 per cent on any house built before 1990. On a new build 5 to 10 per cent is usually enough. The difference is the existing structure.

Does an extension add its cost back in value?
Sometimes, and it depends heavily on the suburb ceiling. A $470,000 second storey in a street where the top sale is $900,000 will not return what it cost. Get a local agent's read on the ceiling before you commit.

Get the numbers before you get the quote

The single best thing you can do before talking to a builder is arrive with your own numbers. Not to argue with the quote, but to understand it, so that when a line item is missing you notice.

The Built Simple app is free to download on iOS and Android, and the calculators work on the phone with no signup. Have a play with them on the way to the site meeting. If you are the builder pricing this work rather than the homeowner buying it, the same app runs your take-offs, quotes and schedule in one place, and there is a free tier to start on.