How Much Does a House Extension Cost? An Honest 2026 Guide
"How much does a house extension cost" is one of those questions where every answer you find online seems to contradict the last one and that's genuinely because the true answer depends on so many variables that a single headline figure is almost always misleading. Still, you need somewhere to start, so here's the most honest breakdown I can give you.
The rough numbers for a house extension in 2026
As a general guide, house extension costs in the UK currently sit between £1,200 and £2,500 per square metre, though the more typical range for a single-storey rear extension is £1,800-£3,000 per square metre, and often £2,200-£3,200 once you account for a reasonable finish level.
Double-storey extensions cost slightly less per square metre — typically £2,000-£2,800 — because you're getting two floors of space out of one roof and one foundation. But the total project cost is usually higher overall, simply because you're building more floor area.
If you're in London or the South East, expect these figures to run 20-40% higher, often £3,000-£5,000 per square metre for a single-storey extension. This is one of the genuine practical advantages of building across Southampton, Hampshire and the wider south coast, where the same specification typically costs meaningfully less.
What actually drives the number up or down in a house extension
Foundations and ground conditions. Difficult access, poor soil, or a sloping site can add significant cost before you've even started building above ground and it's rarely visible in a rough estimate until someone's actually assessed your site.
Structural complexity. Removing load-bearing walls, working around existing drainage or services, and any steel beam requirements all add cost.
Roof design. A simple flat or pitched roof is more cost-effective than a vaulted ceiling or roof lantern, though the latter can transform how a space feels.
Glazing. Large sliding or bifold doors are one of the highest-impact design choices and one of the bigger cost items, budget for this properly rather than as an afterthought.
Finish specification. The figures above cover the structural build. Flooring, internal joinery, lighting design and any kitchen or bathroom fit-out sit on top, and this is often where the biggest variation between two "similar" extensions actually comes from.
Why a single quote can vary so much from another
If you've had two wildly different quotes for what feels like the same project, it's rarely one builder trying to overcharge you, it's more often that the two quotes aren't actually pricing the same specification. One might include a fully finished space, the other a shell ready for you to fit out separately. This is exactly why a clear, detailed specification matters so much before you start comparing numbers, and it's a big part of what a proper design process gives you: clarity on exactly what you're comparing.
Building in a realistic contingency for a house extension
Whatever figure you land on, build in 10-15% contingency, genuinely protected rather than quietly absorbed into the main budget. Extensions involve ground conditions and existing structures that can't always be fully assessed until work is underway, and the projects that stay calm are the ones with room built in to absorb the unexpected.
Getting a number you can actually trust
These figures are a genuinely useful starting point for planning, but your actual house extension cost depends on your specific site, structural requirements and finish level. Anyone giving you a confident final figure without seeing your property is guessing.
If you're planning an extension across Southampton, Hampshire or the wider south coast and want an honest, accurate figure for your own home — from a team that manages design and build together — get in touch and let's talk through your project.
How can working with an Interior Designer at Hinton House Interiors help?
Maybe you're stuck between decisions and just need a second opinion. Maybe it's a single room crying out for a refresh. Or maybe you simply don't have the time to take it all on yourself.
Whatever stage you're at, Hinton House Interiors is here to help. I offer a full design service, including design and build management, as well as one-off design consultations to talk through any dilemmas you're facing.
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Residential Interior Designer based in Southampton, Hampshire, also covering the UK.
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