If you have started pricing a home extension, you have probably run into "cost per square metre" thrown about like it settles the question. It is a handy way to get a rough home extension cost in your head, but it is a starting point, not a fixed price. Two extensions the exact same size can land a long way apart once you factor in what you are building and what your block is like. Here is how the per-m² figure works here in the Illawarra, and where it can quietly move on you.
Extension cost per square metre: the rough ranges
For a ground-floor extension in NSW, most projects sit somewhere around $3,500 to $6,000 per square metre. A second-storey addition usually costs more per m², often $4,500 to $7,500, because you are working above an existing home and the structure has to do more. A kitchen or bathroom inside that footprint pushes the rate up too, since those rooms carry the most fit-out per square metre.
Treat those as a starting point, not a quote. The finishes you choose, the current build climate, and your particular site can move the number in either direction. A small, simple room out the back sits at the lower end. A double-storey addition with a new bathroom and a decent kitchen sits well above it.
Single storey vs double storey
A single-storey, ground-floor extension is usually the cheaper way to add space per square metre. You are building out at ground level, the roof is straightforward, and the trades can get in and out easily.
Going up is a different job. A second-storey addition means the existing structure has to carry the new load, so there is often engineering, footing work, and sometimes strengthening below before anything new goes up. You also lose part of the house to weather during the build, and access is harder. That is why the per-m² rate climbs. The trade-off is that you keep your yard, which matters a lot on the tighter blocks around Wollongong.
What the rate usually includes
When a builder quotes a per-m² figure, it generally covers the core of the build:
- Structure, framing and roofing for the new area
- Windows, doors, insulation and plasterboard
- Standard flooring, painting and basic fit-out
- The trades to tie the new space into your existing home
- Site setup, waste and the day-to-day running of the job
That is the bones of a livable, finished room. It is enough to compare one extension against another at a glance, which is the whole point of the number.
What it usually doesn't include
This is where a lot of the surprises live. A per-m² rate often leaves out:
- Design, engineering and council approval, whether that is complying development (CDC) or a full DA
- Kitchen and bathroom fit-out above a basic standard
- Upgrades to the existing home, like re-roofing or matching old brickwork
- Landscaping, decking, driveways and fencing after the build
- Connecting or upgrading services, and any electrical work the older part of the house needs to meet current rules
None of these are hidden costs. They are just separate lines that a single square-metre figure cannot carry. A fixed-price proposal is where they get spelled out properly, so you can see the real total rather than a rate.
Why your site swings the number
Two identical extensions on two different blocks rarely cost the same, and the ground is usually the reason.
- Slope. A sloping Illawarra or Southern Highlands block often needs more footings, retaining or piering, which the flat-site rate never sees.
- Access. If a truck and trades cannot get near the work, more is done by hand and it takes longer.
- Soil and drainage. Reactive clay or a wet site means more engineering under the slab.
- Tying into an older home. Matching brick, roof lines and floor levels on an established house takes care and time.
- Approvals. Whether your job goes through CDC or a DA depends on your zoning and your block, so confirm the path with your council or a private certifier before you bank on a timeframe.
Getting a real number for your block
The per-m² figure is worth knowing because it helps you sanity-check a budget and rule ideas in or out early. What it cannot do is price your actual home. The only way to get that is to have someone look at your block, your existing house, and what you want to add.
At Ovas Build we work through the design, the council approval and the build together, then put it in front of you as one fixed-price proposal with the inclusions written down. That way the number you are looking at is your number, not an average.
Want a hand with this at your place? Get a free quote or call 0434 037 851.