What a house extension costs in Brisbane, and why

Updated 19 Aug 2026/10 min read/By Manttan Construction · QBCC 15030266

Indicative ranges for residential extension and renovation work

$2,500–$4,500 / m²	Ground-floor extension, new slab and roof tied into the existing house, mid-range finish.
$3,500–$5,500 / m²	Second storey or build-under, including structural upgrades to what is already there.
$25k–$60k	Bathroom or ensuite rebuild, depending on size, tiling and whether the layout moves.
$30k–$70k	Kitchen rebuild, depending on joinery, stone and whether walls or services move.
$3k–$10k	Design, engineering, soil test, energy report and certifier fees on a typical extension.
10%	Contingency we recommend you hold back for what opens up inside an older house.
5%	Our deposit, at every contract level — the rest is claimed against completed stages.

01 — Drivers

What actually drives the price?

Two extensions of identical size can differ by a hundred thousand dollars. The floor area is rarely the reason. These are the variables that actually set the price:

• Access. Can a truck and machine reach the work, or does everything get carried through the house? This is the single most underestimated cost.
• Slope and soil. Fall across the site drives retaining, stepped footings and earthworks; reactive clay drives footing depth and design.
• The existing house. Age, framing condition, termite history and whether the current footings can carry a second storey.
• Services. Moving a kitchen, bathroom or laundry means plumbing and drainage — and possibly an upgraded switchboard or sewer connection.
• Roof and structure. Tying a new roof into an old one, or removing load-bearing walls, needs steel or LVL and engineering.
• Finishes. The same room can be finished for $40k or $140k. This is the part you control most directly.

If your block is steep or tight, read the tight access page — plant that fits through a gate is often the difference between a normal price and a difficult one.

02 — Up or out

Ground floor or second storey — the cost difference

The instinct is that going up must cost more. Usually it does per square metre, but not always overall — because a ground-floor extension consumes land, and land you cannot rebuild.

• Ground floor. Simpler to build and stage, but it eats yard, may hit site cover limits, and needs new footings and slab.
• Second storey. Keeps the garden and often captures a view, but loads the existing structure, needs a stair, and usually means moving out for part of the build.
• Build under. On a high-set house this is the cheapest floor area available — if you have the head height and the stumps are sound.
• Site cover and setbacks. Council limits how much of the block can be built on, which sometimes decides the question for you.
• Existing footings. A second storey lives or dies on whether the current footings and frame can carry it; the engineer answers that, not the builder.

03 — The quote

What a real quote should include

What a real quote contains

A scope you can read: room by room, structural work named, inclusions and exclusions listed, allowances stated with dollar figures, and a stage-based payment schedule. Ours also names the engineer, certifier and the QBCC insurance position.

What vague quotes leave out

Lump sums with no breakdown, "allowances" with no number, no mention of engineering or certification, no site or soil assumptions, and nothing about what happens when something is found inside a wall.

What a fair contract looks like

Written, QBCC-compliant, with a 5% deposit, progress claims tied to completed stages, a variation process in writing, and defect liability spelled out. If a builder wants a large cash deposit, that is your signal to walk.

Our licences, contracts and insurance guide sets out what you are entitled to ask for before you sign anything.

04 — Blowouts

Where extension budgets blow out

Most overruns are not greed, they are discovery. Termite or water damage behind linings, stumps that have to be replaced once the load is taken, old wiring that cannot be legally extended, or previous unpermitted work that has to be brought up to standard before anything new can be approved.

The rest is choice. Selections made after the contract — a better tap, a different stone, a wider door — are variations, and variations always cost more than the same decision made before work started. Decide the finishes early, hold 10% back, and the project stays boring in the best way.

05 — Spending less

How to spend less without wrecking the job

• Keep wet areas where they are. Moving plumbing is expensive; a better bathroom in the same footprint rarely is.
• Simplify the roof. Every extra hip, valley and change of pitch costs money and adds leak risk.
• Build the shell now, finish later. A lock-up second storey with one room fitted out beats a compromised layout you live with forever.
• Choose finishes for the room, not the catalogue. Spend on the surfaces you touch and see; save on what is behind a door.

The cheapest extension is the one designed properly the first time. Redrawing after a certifier or an engineer says no is the most expensive kind of saving there is.

Questions

Extension cost FAQ

Want a real number for your place? Call 0402 847 873 and we will tell you straight.

How much does a 40 square metre extension cost in Brisbane?+

On the 2026 ranges above, roughly $100,000 to $180,000 for ground-floor work at a mid-range finish, before approvals and contingency. Access, slope, soil and the condition of the existing house move that figure, which is why we look before we price.

Is a second storey cheaper than moving house?+

Often, yes, once you add stamp duty, agent fees, moving costs and the premium for a bigger house in the same suburb. It also keeps you in the school catchment and the street you chose.

Why do builders quote so differently for the same job?+

Usually because they are not quoting the same job. One price includes engineering, certification and real allowances; another leaves them out and picks them up as variations. Compare scopes line by line before comparing totals.

How much deposit should a builder ask for?+

In Queensland the deposit is capped by contract level, and we work off 5% across the board. A builder asking for a large upfront payment before work starts is a warning sign.

Extensions & second storeys→ Licences, contracts & insurance→ Second storey vs extension→ All guides→

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