House Extension Cost Calculator & Price Guide
Use our house extension cost calculator to get a quick guide to the cost of a house extension in Australia, whether you are adding a ground-floor room, a larger living area, or a second storey. Typical supply-and-install pricing starts from about AUD 90,000 for a ground-floor room and can reach AUD 400,000 for a second-storey addition on a standard site, before any bathroom or access multipliers are applied.
Estimate house extension cost
Build midpoints for one house extension in Australia. Rates cover a typical design-and-construct job on a flat site. Council fees, a new kitchen, and major structural steel are usually extra unless they are part of the second-storey option.
Estimates are indicative only. For an accurate price, get free quotes from local tradies above.
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House Extension Price Breakdown
Typical job based on 1 extension (Standard site, no wet area). Prices are supply-and-install midpoints — actual quotes will vary.
| Type | Cost per extension (Standard site, no wet area) | Typical 1 extension (Standard site, no wet area) job |
|---|---|---|
| Ground-floor room | $90,000 | $90,000 |
| Living extension | $160,000 | $160,000 |
| Second storey | $400,000 | $400,000 |
What Affects House Extension Cost?
- Extension type: The biggest driver of house extension cost is the type of build. A ground-floor room is priced from AUD 90,000, a living extension from AUD 160,000, and a second-storey extension from AUD 400,000 on a standard site with no wet area.
- Bathroom inclusion: If the extension includes a bathroom, the calculator applies a 1.25 multiplier to the base price. This increases the cost for house extension work because wet areas generally require extra plumbing, fixtures, and finishing trades.
- Site access and slope: Tight access or a sloping site increases the price with a 1.4 multiplier. More difficult sites can add complexity for materials handling, excavation, and construction time for home extension builders.
- Number of extensions: The calculator is based on the number of extensions, so total pricing rises with each additional extension. For typical examples, 1 extension is used as the default input.
House Extension Cost - Frequently Asked Questions
On a standard site with no wet area, a ground-floor room starts at AUD 90,000, a living extension starts at AUD 160,000, and a second storey starts at AUD 400,000. The displayed estimate range is shown at plus or minus 20% around the midpoint. If the extension includes a bathroom, multiply the base rate by 1.25, and if the site has tight access or slope, multiply by 1.4.
A ground-floor room extension is priced at AUD 90,000 per extension on a standard site with no wet area. The displayed estimate range would be approximately AUD 72,000 to AUD 108,000. This option typically suits a bedroom, study, or extra living room of about 15–20 m2.
A living extension is priced at AUD 160,000 per extension on a standard site with no wet area. The displayed estimate range is about AUD 128,000 to AUD 192,000. This usually covers an open living, dining, or family room with a new roof and slab.
A second-storey extension starts at AUD 400,000 per extension on a standard site with no wet area. The displayed estimate range is about AUD 320,000 to AUD 480,000. This type of project generally includes engineering, stairs, and a new roof.
The house extension cost calculator uses a per-extension rate based on the extension type, then applies multipliers where needed. Standard site with no wet area uses x1, including a bathroom uses x1.25, and tight access or a sloping site uses x1.4. The estimate range is then shown at plus or minus 20% around the midpoint, with a minimum job charge of AUD 40,000 if calculations fall below that.
Yes, extension builders usually allow more for difficult access or sloping land because the calculator applies a 1.4 multiplier for these conditions. For example, a living extension priced at AUD 160,000 on a standard site would increase to AUD 224,000 before the estimate range is applied. If a bathroom is also included, the base rate is multiplied by 1.25 as well.
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