Ground floor
How much does a
ground-floor extension
cost in Melbourne?
Between $285,000 and $445,000 for a typical 40 to 45 square metre rear extension with one new wet area, on an inner-suburb house built before 1940. The rate starts at $2,700 per square metre and almost nothing stays at the rate.
The short answer
| Scenario | Assumptions | Range |
|---|---|---|
| Modest addition, outer suburb | 35 m², standard finish, post-1990 house, clear access, no new wet area | $165k – $225k |
| Typical inner-north rear extension | 40 m², mid finish, pre-1940 house, one bathroom | $285k – $385k |
| Larger addition, restricted site | 45 m², mid finish, heritage overlay, no vehicle access | $330k – $445k |
| High-finish, architect-designed | 50 m², high finish, kitchen and bathroom, inner suburb | $520k – $700k |
Base rate: Archicentre Australia CostGuide 2026 · GST inclusive
Site adjustments: AE Index
Where the number comes from
The published rate for new construction and extensions runs from $2,700 to $5,100 per square metre. That figure covers the basic shell and the extended roofline. It is the starting point, not the answer, and four things move it before you have a real number.
Finish level
The single biggest swing available. Standard to high nearly doubles the rate, and it is the one variable entirely within your control. Deciding it early saves redesign fees later.
Wet areas
A kitchen runs $23,000 to $49,000 and a bathroom $17,500 to $35,000, entirely outside the square-metre rate. Most rear extensions include at least one, and many include both.
The house you already own
Restumping, rewiring and re-plumbing to the existing dwelling sit outside the extension rate. On pre-war Melbourne stock, allow around $26,000.
Getting materials to the back
A site with clear side access is cheaper to build on than an identical site without it — commonly by 7 to 15%. It is rarely itemised in a quote and always inside the price.
Ground floor versus second storey
A ground-floor addition is usually the cheaper way to add space, because new structure sits on new footings rather than on a house that was never designed to carry it. If you have the land for it, extending outward almost always beats extending upward on cost per square metre.
The trade-off is garden. On the narrow inner-north and inner-city blocks where most Melbourne renovation happens, a rear extension consumes the yard, and that is often what pushes people upward despite the higher rate.
What is not in the rate
Professional fees and permits — architect, structural engineer, building surveyor, permit levy and energy rating assessment — add roughly 25% on top of the construction cost. Contingency is a further 10 to 15%. Neither appears in a builder's construction quote, and both are compulsory in practice.
Excluded entirely: landscaping and external works, appliances, furniture and window furnishings, council planning application fees, driveway reinstatement, and anything discovered once walls are opened.
Get an indicative range
Ground-floor extension is already selected. Eight questions, and you will see every line of the calculation.
Common questions
Is a ground-floor extension cheaper than a second storey?
Usually yes, per square metre. New structure on new footings avoids the cost of upgrading an existing house to carry load. The trade-off is that you lose yard, which is why constrained inner-city blocks often go upward despite the higher rate.
Do I need a planning permit for a rear extension?
It depends on the overlay on your title, the site coverage, and how close you build to boundaries. Many single-storey rear additions in Melbourne proceed on a building permit alone, but a heritage overlay or a boundary issue will usually trigger a planning application. Check your title before briefing a designer.
How much does a 40 square metre extension cost?
On an inner-suburb pre-1940 house with a mid-range finish and one new wet area, expect $285,000 to $385,000 all in, including professional fees and contingency. On a post-1990 outer-suburb house with clear access and no wet area, the same floor area can land closer to $190,000.
Why is the published square-metre rate so much lower than my quote?
Published rates cover the shell and roofline only. Wet areas, work to the existing house, professional fees, permits and contingency all sit outside them. Applied straight, a published rate will understate a real project substantially.
Related
When you are ready to build
AE Index is not a building company. When a project is ready to be priced properly, we pass it to a builder from our verified network — who will walk the site, inspect what is under the floor, and put a fixed price in writing. We are paid a fee by them on work that proceeds, and you are under no obligation to use them.
- Current domestic builder registration, confirmed with the Victorian Building Authority
- Domestic building insurance current and sighted
- Two completed projects contacted directly — not supplied testimonials
- Fixed-price contracts with a written variations policy
- Re-checked annually, and removed from the network on any lapse
Last updated 28 July 2026