What an extension costs per square metre, and what drives it

You've scrolled through a few builder quotes and noticed the prices per square metre vary wildly. One builder came in at what feels like half the cost of another. The difference isn't always a bargain—sometimes it reflects something specific missing from the cheaper quote, which is worth identifying rather than assuming. Understanding what drives cost, and what a low estimate quietly omits, stops you from comparing apples to chainsaws.

What's bundled into that square-metre number

A builder's quote per square metre should cover labour, materials, site management and preliminaries (like temporary site setup, waste removal, safety compliance). But "should" is doing a lot of lifting. Some quotes carve out structural work as a separate line. Others exclude roof work, windows, or finishes. A few leave out site setup entirely—then bill you later, which is a verifiable gap you can catch by asking upfront whether temporary ablution facilities and dust protection are included.

The honest quotes itemise this. They show you labour as a percentage, materials as another, and flag what's excluded. A single price-per-metre with no breakdown makes it harder to see what's included, whatever the price—ask for an itemised version so you can check for gaps yourself. When you compare two quotes, you're not comparing price; you're comparing what each builder included in their calculation. A lower square-metre rate might mean they've carved out electrical rough-in, plumbing, or concrete foundations. Ask explicitly: "What's included in your per-metre quote?" Write down the answer. You'll spot the gaps.

The hidden cost drivers nobody mentions upfront

Three things shift your per-metre cost dramatically, yet few builders flag them early. First is site access. An extension on a corner stand with a wide driveway costs less than one squeezed onto a narrow plot in a dense suburb. Moving equipment, delivering materials, and managing traffic around the build site takes time and ingenuity. A builder working on a tight urban site will quote higher per square metre because logistics are harder.

Second is existing conditions. If your house sits on unstable fill, or the foundation is shallow, or there's a tricky slope, the builder needs more groundwork before they even start the extension itself. Some quotes assume a straightforward build; others budget for soil testing, shoring up, or drainage works. You won't see "unknown ground" as a line item—it gets absorbed into labour or marked as contingency. Ask: "Have you visited the site and checked the existing foundation and soil?" If they haven't, ask how they arrived at the figure without a site visit—a considered answer (such as a standard allowance pending inspection) is different from no answer at all.

Third is specification detail. An extension with basic finish—one plaster coat, simple paint, standard roof tiles—costs less per square metre than one with rendered feature walls, high-end finishes, or a pitched roof with dormers. The builder's quote reflects what you asked for. If two builders quoted at different rates, check whether one included a flat ceiling and the other a vaulted one, or whether one specified economy tiles and the other premium ones. The per-metre rate is only meaningful when the specification is identical.

Making quotes actually comparable

Request quotes in the same format. Ask each builder to list labour, materials, site costs, and structural/engineering extras separately. Specify the exact finishes you want—roof type, wall treatment, ceiling height, floor covering. Ask what's excluded and what's contingency. A contingency line isn't a hidden cost; it's insurance against the unknowns (that cracked foundation, unexpected rot in the existing wall, a utility line in the wrong place).

When you spot a quote that's noticeably cheaper, email the builder and ask: "What's not included in this figure?" The answer tells you whether it's a genuine efficiency or a trap. A builder who's mastered their supply chain and labour roster might deliver better value legitimately. Whatever a builder's price, watch for specific verifiable behaviour—no written scope, vague answers about who's doing the work, or a pattern of add-on billing once the project starts—since these are the real signs of risk, not the price itself.

Verified builders on Strove include detailed breakdowns in their quotes and flag what's contingent. Comparing their itemised figures—not just the headline number—shows you the real cost and what you're actually paying for.