Planning a house extension? The first question everyone asks is: how much is it actually going to cost?
Here’s the honest answer for 2026, based on real UK project costs — not vague estimates from five years ago.
Average House Extension Costs in 2026
These are ballpark figures based on typical UK builds. Your actual cost will depend on location, spec, and complexity — but this gives you a realistic starting point.
Single Storey Rear Extension
£1,500 – £2,500 per m²
A typical 4m × 5m (20m²) single storey extension costs between £30,000 and £50,000. This is the most common type of extension in the UK — usually a kitchen-diner or open-plan living space with bi-fold or sliding doors to the garden.
Double Storey Extension
£1,800 – £3,000 per m²
Going up as well as out typically costs £50,000 to £90,000 for a standard two-storey rear extension. You get roughly 50-60% more space than a single storey for only about 40% more cost — making it significantly better value per square metre.
Kitchen Extension
£35,000 – £70,000
Kitchen extensions tend to cost more per m² because the fit-out is expensive. A new kitchen alone can be £10,000-£25,000, plus plumbing, electrics, flooring, and often underfloor heating. Budget for the extension structure AND the kitchen separately.
Wrap-Around Extension
£60,000 – £120,000
Combining a rear and side extension creates a large open-plan space, but costs more due to the additional foundations, walls, and roofing. These projects often require more structural steel too.
What Affects the Cost?
No two extensions cost the same. Here’s what moves the needle:
- Location — London and the South East can be 30-50% more expensive than Wales or the North. Labour costs vary hugely by region.
- Size and complexity — A simple rectangular single storey is straightforward. Add a vaulted ceiling, structural glazing, or a basement and costs climb fast.
- Specification — Aluminium bi-fold doors vs uPVC. Quartz worktops vs laminate. Underfloor heating vs radiators. Every upgrade adds up.
- Ground conditions — Clay soil, high water tables, tree roots, or sloping sites all increase foundation costs.
- Access — Tight access for deliveries and machinery means more manual labour and higher costs.
How to Keep Costs Under Control
The biggest money-saving decision happens before a single brick is laid: get the design right first.
- Get a build cost analysis at design stage. At Arkiplan, we provide a detailed cost breakdown before you commit — so you know what you’re getting into before applying for planning.
- Design efficiently. A simple footprint with standard roof pitches costs far less to build than complex shapes with multiple roof junctions.
- Fix your architectural costs upfront. Traditional architects often charge percentage-based fees that creep up. We charge a fixed price from day one — no surprises.
- Get multiple builder quotes. Always get at least three quotes. The spread can be 30-40% between cheapest and most expensive.
The Hidden Costs People Forget
The build cost is only part of the picture. Budget for these too:
- Architectural drawings — £1,195 to £4,000+ depending on the service (we start at £1,195 for a complete package)
- Planning application fee — £258 for a householder application in England (2026)
- Building regulations fee — £400 to £1,200 depending on your council and project size
- Structural engineer — £500 to £2,000 for calculations (included in our Premium package)
- Party wall agreement — £700 to £1,500 per neighbour if you’re building on or near the boundary
- Utilities — Moving drains, gas pipes, or electrical supplies can add £2,000-£5,000
Get a Realistic Quote Before You Start
The best way to avoid cost surprises? Get proper architectural drawings and a build cost analysis done first.
At Arkiplan, we handle everything — drawings, planning applications, building control submissions, and structural calculations — all for a fixed, transparent price. No spiralling fees, no hidden extras.
We’ve completed over 10,000 projects across England and Wales, and every one started with a free instant quote.