An extension is the natural answer when a house is in the right spot but has run out of room. The big question is what it'll cost, and like most building work, the honest answer is a range that depends on your home and your block. This guide walks through what drives the price and gives some rough figures, so you can plan with realistic numbers. The Ovas Build team builds extensions across Wollongong, the Illawarra and the Southern Highlands.
A note up front: every home is different, prices move, and the only real figure is a quote on your job. Treat the ranges here as a rough guide, not a quote.
Rough per-square-metre ranges
As a rough guide, a ground-floor extension in the Illawarra in 2026 is often costed in the low-to-mid thousands per square metre, but that figure moves a long way depending on what's in the new space. A simple addition like a bedroom or rumpus, with no plumbing and a standard finish, sits at the lower end. A new kitchen, bathroom or laundry, full of plumbing, electrical and tiling, sits well above it because there's far more in the same floor area.
So a 30 square metre living extension and a 30 square metre extension that includes a new bathroom can come in at very different totals, even though the footprint's the same. That's why per-square-metre figures are only ever a starting point.
What drives the price
Footings and the ground. What's under the extension matters as much as what's on top. A flat block with good soil is cheap to found. A sloping block, reactive clay, or poor ground means engineered footings and sometimes a suspended floor, which adds cost before a wall goes up. Plenty of Illawarra blocks fall away off the back, so this is a common factor down here.
The roofline tie-in. Joining the new roof to the old one and keeping it watertight is one of the fiddlier parts of any extension. Depending on the existing roof's pitch and design, this can be straightforward or it can mean reworking a section of the existing roof. It's a part of the job we look at carefully before pricing, because it's where surprises hide.
Site access. If materials and trades can get to where the extension's going easily, the build runs efficiently. If everything has to come through the house or down a narrow side passage, that's extra time and handling.
Finishes. The same shell can carry a budget fit-out or a high-end one. Flooring, joinery, benchtops, tiling and fixtures are where a lot of the variation sits, and it's the part you have the most control over once the structure's set.
Matching the existing home. Brickwork, cladding and windows that match the existing house cost more than the cheapest available, but they're what stop the extension looking bolted on. We think it's worth doing properly.
Out or up?
Extending out on the ground floor is usually cheaper per square metre than adding a second storey, because going up means structural work to carry the new floor and often strengthening the existing footings and walls. But not every block has the land to extend out, and a second storey keeps your yard. We go through the figures on our guide to the cost of adding a second storey if you're weighing the two up.
A fixed price, no surprises
We give a fixed-price proposal before any work starts, with the design, engineering and approval included, so the number is set before the first footing's dug. Because we handle the framing, roofing and finish work in-house, the tricky junction between old and new is managed by one crew rather than passed between trades. We run tidy sites and use coastal-grade materials that suit the conditions here.
For a real figure on your home, the only way is a look on site. Read how we approach home extensions, then call us on 0434 037 851 or use the contact page for a free, no-pressure quote.
Common questions
- Is an extension priced per square metre?
- It's a useful starting point, but it's only ever a rough guide. The per-square-metre figure swings a lot depending on whether you're adding a simple bedroom or a kitchen and bathroom full of plumbing and finishes, and on what's involved tying into the existing home. We use it to ballpark, then quote the actual job properly.
- Why does tying into the existing house add cost?
- Because the join is the tricky part. The new roofline has to meet the old one and stay watertight, the walls and floors have to line up, and the brickwork or cladding has to match. There's often unexpected work where the new structure meets the old, which is why we look closely at the junction before pricing.
- Does an extension need approval, and is that in the price?
- Almost always, because adding floor area is structural. It runs as either a Complying Development Certificate or a DA, depending on the block and design. We include the design, engineering and approval in our fixed-price proposal so it's not a surprise cost on top.
- Is it cheaper to extend out or build up?
- Extending out on the ground is usually cheaper per square metre, because going up adds structural work to carry the new floor and often means strengthening the existing footings and walls. But out needs spare land and up doesn't, so the right answer depends on your block. We cover this on our second-storey cost guide.