How Much Does a House Extension Cost in London? (2026 Guide)

If you’re considering a house extension in London, one of the first questions you’ll ask is: how much will it cost? In 2026, extension prices continue to reflect high demand, labour shortages, and rising material costs across the capital.

This guide breaks down everything you need to know — from average costs and extension types to hidden fees and money-saving tips — so you can budget with confidence.

Average House Extension Costs in London (2026)

As a general rule, house extension costs in London are higher than the UK average. Here’s what you can expect:

• Basic extension: £2,200 – £2,800 per m²

• Mid-range extension: £2,800 – £3,800 per m²

• High-end extension: £3,800 – £5,500+ per m²

Typical Total Costs:

• Single-storey extension (20m²): £50,000 – £110,000

• Double-storey extension (40m² total): £100,000 – £220,000

• Rear kitchen extension: £60,000 – £150,000

• Side return extension: £45,000 – £100,000

Cost by Extension Type

1. Single-Storey Extension

The most common option for London homes.

• Ideal for kitchens, dining rooms, or open-plan living

• Lower structural complexity

• Faster build time

Cost: £50,000 – £110,000

2. Double-Storey Extension

Adds space upstairs and downstairs, making it cost-effective per m².

• Requires more structural work

• Higher planning scrutiny

Cost: £100,000 – £220,000

3. Side Return Extension

Popular with Victorian and terraced houses in London.

• Makes use of unused alley space

• Great for kitchen expansions

Cost: £45,000 – £100,000

4. Wrap-Around Extension

Combines rear and side return extensions.

• Maximises ground floor space

• Higher complexity and cost

Cost: £90,000 – £250,000+

Hidden Costs to Consider

Many homeowners underestimate the extra costs beyond construction. These can add 15–30% to your budget:

• Architect fees: £2,000 – £6,000

• Planning permission: £200 – £500

• Structural engineer: £500 – £2,000

• Party wall agreements: £1,000 – £3,000

• Building regulations: £500 – £1,500

• VAT (20%) on most work

What Drives Costs Higher in London?

Several London-specific factors push prices up:

• Limited access (especially in terraced streets)

• Parking restrictions and permits

• Higher labour costs

• Structural challenges in older properties

• Premium finishes and bespoke designs

Cost of Key Materials (2026 Snapshot)

• Brickwork: £600 – £1,200 per m²

• Steel beams (RSJs): £1,500 – £4,000

• Bi-fold doors: £2,500 – £7,000+

• Rooflights/skylights: £800 – £2,500 each

Material prices fluctuate, so always allow a contingency budget of at least 10–15%.

How to Reduce Extension Costs

You don’t have to overspend. Here are smart ways to save:

• Keep the design simple (rectangular layouts cost less)

• Avoid major structural changes where possible

• Use standard-sized windows and doors

• Plan early to avoid costly changes mid-build

• Work with experienced local builders

Does an Extension Add Value?

Yes — in London, a well-designed extension can increase property value by 15–25%, especially if it improves:

• Kitchen space

• Natural light

• Layout and flow

In many cases, the added value can outweigh the cost.

Why Choose RF Trojan?

At RF Trojan, we specialise in high-quality house extensions across London. Our team handles everything from design to completion, ensuring your project stays on time and on budget.

What sets us apart:

• Transparent pricing with no hidden costs

• Experienced builders and project managers

• Tailored designs to suit your home

• Full compliance with UK building regulations

Areas We Cover

• North London

• South London

• East London

• West London

• Greater London boroughs

Get a Quote

Planning a house extension in London? Contact RF Trojan today for a consultation and detailed quote tailored to your project.

FAQs

How long does a house extension take?

• Single-storey: 10–16 weeks

• Double-storey: 16–24 weeks

Do I need planning permission?

Not always. Some extensions fall under permitted development, but it depends on size and location.

What’s the cheapest type of extension?

A simple single-storey rear extension is usually the most affordable option.

Final Thoughts

House extension costs in London vary widely depending on size, design, and complexity. In 2026, most homeowners should budget £2,500–£4,000 per m² for a realistic project.

With proper planning and the right builder, an extension can transform your home — and significantly increase its value.

Need expert advice? RF Trojan is here to help you build smarter, not just bigger.