How much does a home extension cost in Adelaide in 2026?

Current published guides put a ground-floor or single-storey home extension at roughly $2,000 to $4,500 per square metre, and a second-storey addition in Adelaide at roughly $3,500 to $5,500 per square metre, with premium finishes going higher. Those are third-party ranges, not our prices. Where your extension lands inside (or outside) them depends on size, wet areas, structure, the site and the finishes you pick, and the only way to pin it down is a measured, itemised quote.

Below we break down what the published ranges say, what pushes a house extension cost up or down, and the costs people tend to forget. We build home extensions, house additions and second storeys across Adelaide's north, so this is the same conversation we have on site visits in Gawler, Angle Vale, Elizabeth and Riverlea Park.

What the published cost ranges say

Two things are worth reading closely. Templeton says its Adelaide figures include design, engineering, approvals, construction and standard finishes, so not every range is measuring the same thing. And Budget Direct notes a second storey can cost up to 50% more than building out on the ground floor, depending on how much structural work the existing house needs.

To put the numbers in context: a 30 m² family room at Templeton's single-storey range works out at $84,000 to $135,000. That is simple arithmetic on a published range, not a quote, but it shows why the per-square-metre rate matters less than what is going into those square metres.

What moves the price of a house extension

1. Size, and why small extensions cost more per square metre

Design, engineering, approvals, site set-up and tying into the existing roof cost much the same whether you add 15 m² or 40 m². On a small addition those fixed costs are spread over fewer square metres, so the rate per m² goes up even though the total is lower.

2. Wet areas

A new bathroom, ensuite, laundry or kitchen brings plumbing, waterproofing, tiling, fixtures and more electrical work into the extension. Budget Direct makes the same point: wet rooms cost more per square metre than bedrooms or living space. If you are adding an ensuite, read our guide to bathroom waterproofing rules in SA.

3. Going up versus going out

A ground-floor extension needs new footings and a roof. A second storey needs an engineer to confirm the existing footings and walls can carry the load, a staircase, and a roof that comes off and goes back on. We compare the two in detail in ground-floor vs second-storey extensions.

4. The join between old and new

This is where extensions most often go wrong, and where a lot of the cost hides. Matching brickwork and rooflines, lining up floor levels, and opening up the back of the house all take time. Removing a load-bearing wall means an engineered beam. The photo at the top of this page shows that kind of stage on one of our jobs: a wall stripped back to its frame while the layout is opened up.

5. Your block and your soil

Much of northern Adelaide sits on reactive clay that swells when wet and shrinks when dry. New footings are designed from a soil test and site classification, and a more reactive site means deeper or stiffer footings. Slope, tight side access, trees and any sewer or stormwater lines that need moving add cost too. The SA Government's Choosing a builder page notes builders can ask you to pay up front for things like council approval and soil testing, which is why those items are often billed early.

6. The age of the house

SafeWork SA says homes built or renovated before the mid-1980s are highly likely to contain asbestos, and homes from the mid-1980s to 1990 are likely to. Eaves, wall sheeting, bathrooms, laundries and old floor coverings are common spots. Testing and safe removal of anything the extension disturbs needs to be in the budget (SafeWork SA guide).

7. Approvals, consultants and insurance

Expect drafting or design fees, structural engineering, planning and building consent fees, and building indemnity insurance. In SA, builders must take out building indemnity insurance for domestic work that needs council approval and costs $20,000 or more, and work cannot start until you and the council have the certificate (SA.GOV.AU). Our guide to approvals for home extensions in SA covers the process step by step.

8. Finishes

Flooring, joinery, benchtops, windows and doors, lighting and tapware are the biggest swing in any extension. Two extensions with an identical floor plan can be tens of thousands of dollars apart purely on selections.

Costs people forget to budget for

• Soil test, engineering and drafting before anything is built
• Council or accredited professional fees for planning and building consent (not included in our quotes unless expressly stated)
• Electrical upgrades, such as a switchboard that can't take the extra load
• Making good: patching, painting and flooring transitions in the existing rooms next to the extension
• Landscaping, paths and fencing after the build
• Window furnishings, and somewhere else to live if the works make the house unliveable for a stretch

How to budget sensibly

1. Decide what problem the extension solves (another bedroom, a bigger kitchen, a second living zone). It keeps the size honest.
2. Get the site looked at in person. Access, levels, the existing roof and the soil change the price more than any online calculator can.
3. Ask for an itemised quote, so you can see where the money goes and what is excluded.
4. Keep a contingency for what nobody can see until walls are opened.
5. Check the deposit. For contracts of $20,000 or more, the SA Government says a deposit should be no more than 5% of the total (SA.GOV.AU).

How we price extensions

We don't quote extensions over the phone, because a figure without a site visit is usually wrong. We come out, measure up, talk through whether to extend out, go up or rebuild, and then give you a detailed, transparent quote with no hidden costs. Design, engineering and approvals are managed as part of the job, and electrical work is done in-house by ASP Electrical Installations. See our home extensions page for the full service, or our work in Gawler and Angle Vale.

Common questions

Related pages

• Home Extensions Adelaide
• Builder in Gawler
• Builder in Angle Vale
• Contact ASP Build & Co for a free quote