How Much Does a House Extension Cost in the UK? (2025 Complete Guide)

Planning a house extension is one of the most exciting yet daunting home improvement decisions you can make. The most common question we hear at Fortis Construction is: "How much will it cost?" The honest answer is — it depends. But this guide gives you real figures to plan around.

Average House Extension Costs in the UK (2025)

Here are typical costs for the most common extension types. These include materials and labour but exclude planning fees, architect drawings, and VAT:

Extension Type	Average Cost	Typical Timeline
Single Storey Rear Extension (3×4m)	£25,000 – £45,000	8–14 weeks
Double Storey Extension	£45,000 – £85,000	16–24 weeks
Side Return Extension	£30,000 – £55,000	10–16 weeks
Wrap-Around Extension	£55,000 – £100,000	18–28 weeks
Garage Conversion	£10,000 – £25,000	4–8 weeks

What Affects the Cost of a House Extension?

No two extensions are the same. The following factors will significantly influence your final bill:

• Size and footprint: Every square metre typically costs £1,500–£3,500 to build, depending on quality and complexity.
• Specification level: A basic flat-roof extension costs far less than a glazed lantern roof with underfloor heating and bi-fold doors.
• Ground conditions: Poor soil, tree roots, or nearby drainage can dramatically increase foundation costs.
• Location: Costs in Hampshire and Surrey tend to be 10–20% higher than the UK national average due to demand and labour rates.
• Structural complexity: Removing load-bearing walls to create open-plan space requires steel beams, which adds £2,000–£8,000+.
• Finishes and fixtures: A premium kitchen or bifold doors could add £15,000–£40,000 on their own.

Breaking Down the Costs

Professional Fees (10–15% of build cost)

You'll need an architect or architectural designer for drawings (£1,500–£5,000+), a structural engineer if load-bearing walls are involved (£500–£1,500), and potentially a party wall surveyor if you're building near a neighbour's boundary.

Planning & Building Control (£500–£2,000)

If your project requires planning permission, expect to pay around £206 for the application fee in England. Building regulations approval (which all extensions need) typically costs £500–£1,000 depending on your local authority.

Labour (40–60% of total cost)

Labour is usually the biggest cost. In Hampshire and Surrey, skilled trades typically charge:

• Bricklayers: £200–£300/day
• Carpenters/joiners: £180–£280/day
• Electricians: £200–£350/day
• Plumbers: £200–£350/day
• Plasterers: £180–£260/day

5 Tips to Reduce Extension Costs

1. Get three detailed quotes — not ballpark figures. A detailed breakdown lets you compare like for like.
2. Avoid scope creep — decide exactly what you want before work begins. Changes mid-build are expensive.
3. Choose permitted development — working within PD limits avoids £5,000–£10,000 in planning fees and delays.
4. Use standard sizes — bespoke steelwork, windows, and doors cost significantly more than standard products.
5. Plan your fit-out separately — getting the shell built first and fitting out yourself can save 20–30%.

Is a House Extension Worth It?

Generally, yes — especially in Hampshire and Surrey where property values are strong. A well-designed single storey extension typically adds 5–8% to property value; a double storey extension can add 10–20%. The key is ensuring the project cost doesn't exceed the value it adds.