What Does a Home Extension Really Cost?

A good-quality single-storey extension costs roughly £2,000–£3,000+ per m², so a 20m² kitchen extension typically lands between £40,000 and £60,000 for the build alone—before units and finishes. On top of that come design, structural, building control and party wall fees, VAT, and a 10-15% contingency. Here's the honest breakdown.

Let's talk money. It's the question on every homeowner's mind, and the one most architectural designers dance around: "What's this actually going to cost me?"

No vague answers here. Let's break down what really drives the cost of a home extension — so you can budget with confidence and avoid the nasty surprises that derail so many projects.

The Big Cost Drivers

Not all extensions are created equal. Here's what moves the needle most:

• Size and footprint — Bigger isn't always proportionally pricier, but more floor area means more materials and labour.
• Single vs. double storey — A second storey often adds surprising value, because you're sharing the cost of foundations and roof across more space.
• Ground conditions — Tricky soil, slopes, or trees nearby can push up groundwork costs.
• Glazing and finishes — Big bifold doors and roof lanterns look stunning but add up fast.
• Kitchen and bathroom fit-out — These rooms carry the highest per-square-metre cost.

Realistic Cost Ranges

As a rough guide for a good-quality extension:

• Basic single-storey: £2,000–£2,500 per m²
• Higher-spec single-storey: £2,500–£3,000+ per m²
• Double-storey: often better value per m² because foundations and roof are shared

So a 20m² kitchen extension might land somewhere between £40,000 and £60,000 for the build alone — before kitchen units, furniture, and the finishing touches.

The Costs People Forget

This is where budgets blow up. Don't forget:

• Design and planning fees
• Structural engineer's calculations
• Building control fees
• Party wall agreements (if you're near a boundary)
• VAT
• A contingency (always keep 10–15% spare)

That contingency isn't optional. Every honest builder will tell you: something always comes up.

How to Keep Costs Under Control

• Get the design nailed down before you start. Changes mid-build are the number one budget killer.
• Be realistic about spec. Champagne taste on a lemonade budget leads to disappointment.
• Choose a builder on trust and track record, not just the lowest quote.
• Build in a contingency and protect it.

The Honest Truth

Here's what we tell every client: the cheapest quote is rarely the cheapest project. Corners get cut, extras pile up, and you end up paying more in the long run. Pay for quality where it counts, and budget realistically from the start.

Let's Get Your Numbers Right

Want a realistic, honest cost estimate for your project — with no sales spin? We'll help you understand exactly what your extension is likely to cost, and how to make your budget work hardest.