The house extension cost question usually arrives at the kitchen table, somewhere between "we love this street" and "we cannot keep living like this". The honest answer is a set of ranges, not a rate, because an extension is surgery on an existing building, and no two patients match. This guide from Sakar Custom Homes sets out realistic Australian numbers for 2026, what moves them, and how Adelaide families keep an extension budget from drifting.
House extension cost at a glance
For 2026, plan on $3,000 to $5,500 per square metre for ground floor extensions and $4,000 to $7,000+ for second storey additions. A 60 square metre ground floor extension commonly lands between $180,000 and $330,000; substantial whole home projects run well beyond. Add design, engineering and approvals at $15,000 to $40,000.
House extension cost per square metre in Australia
Extensions cost more per square metre than new homes, and it helps to know why rather than just resent it. A new home is built in open space in an efficient sequence; an extension is stitched onto a living building. Trades work around your family, floors and rooflines must marry perfectly to the old structure, and demolition of the joining wall happens with your furniture on the other side. That is why $3,000 to $5,500 per square metre is the honest ground floor range while project home builders advertise less for greenfield space. Within the range, the position is set by structure, wet areas and finishes. A simple family room lands low; a new kitchen wing with cathedral ceilings lands high.
Ground floor versus second storey house extension cost
Building out is cheaper per metre than building up, when you have the land. Ground floor extensions run $3,000 to $5,500 per square metre; going up runs $4,000 to $7,000 or more, because the existing structure must be engineered to carry the new level, the roof comes off, and a staircase consumes floor area on both levels. Building up wins when land is tight, when the backyard is worth keeping, or when views and light live at the second level. Our dedicated guide to second storey addition cost runs those numbers in detail, and the Sakar extensions and renovations service prices both paths against your actual house so the comparison is real rather than theoretical.
What drives a house extension cost up or down
Five factors set your position in the range. Structure: removing load bearing walls to connect old and new means steel, and steel means engineering. Wet areas: a bathroom inside the extension adds waterproofing, tiling and services density; a kitchen adds joinery and appliances. The junction: matching floor levels, rooflines and external materials is craft, and craft is time. Access: a tight side path means materials move by hand. And the age of the house: older homes hide surprises, from undersized footings to wiring that fails inspection the moment it is touched. The Australian Bureau of Statistics tracks renovation activity nationally at abs.gov.au, and the trend is steadily upward, which is one more reason quality trades are booked ahead.
The house extension cost items people forget
The contract price is not the project price. Design, engineering and approval fees run $15,000 to $40,000 before a slab is poured. If the ceiling comes down or the wiring is old, compliance upgrades to smoke alarms and switchboards follow automatically. New space wants furnishing, window coverings and often landscaping where the yard was a work site. And if the kitchen or laundry is out of action for months, factor the cost of living around that. None of these should stop you; all of them belong in the budget on day one, which is exactly how the Sakar Custom Homes process builds an extension budget, complete before contract rather than corrected after it.
An extension is surgery on a building you love. Choose the surgeon on judgement, not on the lowest quote.
House extension cost versus the cost of moving
The alternative to extending is buying the bigger house, and it carries its own invoice: stamp duty, agent commission on your sale, conveyancing twice, and the removalists, commonly $80,000 to $150,000 on established Adelaide property before you gain a single square metre. Extending spends that money on fabric you keep, in the street you chose, near the schools that anchor your week. The honest exceptions: if your block cannot absorb the space you need, or the house is structurally poor, moving or a knockdown rebuild may serve better. Sakar Homes will tell you which side of the line your house sits on in the first conversation, because a builder should be a source of judgement before it is a source of quotes.
How to keep a house extension cost under control
Three disciplines do most of the work. Investigate first: a structural inspection and measured drawings of the existing house convert the worst surprises into line items before anyone prices anything. Design completely: every tile, tap and window chosen before contract is a price you locked; every one chosen after is a variation. And insist on a fixed price with named allowances you have checked against real products. The right interrogation for any builder is in our guide to questions to ask a builder. Ask them all; a builder who welcomes the list is a builder whose number will hold.
House extension cost examples for three common projects
Worked examples anchor the ranges better than rates do. A 25 square metre family room across the back of a 1970s house, no plumbing, decent access: roughly $85,000 to $140,000, sitting at the low end of house extension cost territory because it is one simple structure with one junction to the old roof. A 55 square metre kitchen and living wing with a new laundry: $200,000 to $320,000, because the two wet areas, the joinery and the steel needed to open the back of the house all sit in the most expensive part of the range. And a 90 square metre second storey with two bedrooms and a bathroom: $360,000 to $600,000 or more, carrying the structural premium of building up.
Notice what moves between the examples: not the floor area so much as the density of services, structure and joinery inside it. That is the entire logic of house extension cost, and it is why two extensions of identical size can be $150,000 apart with both builders pricing honestly. When you receive a quote, ask which example it most resembles and why. A builder who can place your project on this spectrum, and explain the placement, is pricing from understanding; a builder who quotes a flat rate per square metre for an extension is pricing from hope. Sakar Custom Homes prices from measured drawings of your actual house, which is the only version of the number worth signing.
Get a real house extension cost for your home
Ranges end where your floor plan begins. Sakar Custom Homes offers an unhurried first conversation at your home: what you need, what the structure will allow, and an honest range for the paths open to you, extend out, build up, or occasionally the braver rebuild conversation. Contact the Sakar Homes team or tell us about your project online, and start from a number your family can actually plan around.
House extension cost FAQs
How much does a house extension cost in Australia?
In 2026, ground floor extensions typically run $3,000 to $5,500 per square metre, and second storey additions $4,000 to $7,000 or more, so a 60 square metre ground floor extension commonly lands between $180,000 and $330,000 once wet areas and finishes are counted. Kitchens and bathrooms inside the new space push the rate to the upper end.
House extension cost timelines: how long does a house extension take?
Allow two to four months for design and approvals, then four to eight months of construction for a substantial extension. Projects that open the roof or restructure walls sit at the longer end. Sakar Custom Homes sequences the work to keep your home liveable and weathertight throughout.
Does the house extension cost include council approval?
It should, and it is worth checking. Approval is almost always required: extensions in South Australia are assessed through PlanSA, and character overlays or heritage areas add requirements. Sakar Homes includes preparing and lodging the application in the service, so approval risk is managed before construction pricing is locked.
Is the house extension cost cheaper than moving house?
Once stamp duty, agent fees and moving costs are counted, often $80,000 to $150,000 on established property, a well planned extension frequently wins, and the money becomes fabric you keep rather than tax you spend. The deciding factor is whether your block can absorb the space you need.
How much does a small house extension cost?
A modest 20 to 30 square metre ground floor addition, a bedroom or a family room without plumbing, typically costs $70,000 to $160,000 depending on structure, access and finishes. Wet areas change the mathematics quickly, because bathrooms and kitchens are the densest square metres in any home.
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