House Extension in London: Costs, Timeline & What to Expect in 2026

Expanding your home is one of the smartest ways to increase living space and property value — especially in London, where moving to a bigger house is often significantly more expensive than extending your current one.

If you're planning a house extension in 2026, here’s everything you need to know about costs, timeline and what to expect.

How Much Does a House Extension Cost in London in 2026?

Extension costs vary depending on size, complexity and specification. However, average 2026 prices in London are:

• Rear extension (single-storey): £2,200 – £3,500 per m²

• Double-storey extension: £2,000 – £3,000 per m²

• Side return extension: £2,500 – £3,500 per m²

• Wrap-around extension: £3,000+ per m²

High-end finishes, bespoke joinery, structural steel, glazing systems and underfloor heating will increase the budget.

For example:A 25m² single-storey rear extension may cost between £55,000 and £90,000, depending on specification.

What Affects the Final Cost?

Several key factors influence pricing:

• Structural complexity

• Ground conditions

• Planning requirements

• Access to the property

• Specification level

• Kitchen or bathroom installation

• Bespoke joinery and finishing

Many homeowners underestimate finishing costs — flooring, lighting, decoration and custom carpentry can represent a significant portion of the total investment.

Timeline: How Long Does It Take?

Here’s a realistic timeline for a London extension project:

1. Design & Planning – 6 to 12 weeks

Includes architectural drawings, structural calculations and planning approval (if required).

2. Pre-Construction – 3 to 6 weeks

Party wall agreements, building control setup and site preparation.

3. Construction – 12 to 20 weeks

Depending on complexity and size.

In total, you should allow 5 to 8 months from idea to completion.

Do You Need Planning Permission?

Not all extensions require full planning permission. Many fall under Permitted Development, but this depends on:

• Property type

• Conservation area status

• Previous extensions

• Size of the proposed extension

Professional advice at the early stage can prevent costly mistakes and delays.

Is It Worth It?

In most London areas, a well-designed extension can increase property value by 15–25%.

But beyond financial return, it improves lifestyle:

• Larger kitchen

• Open-plan living

• Better natural light

• More functional layout

A carefully planned extension is not just extra space — it’s a long-term investment.

Final Thoughts

A house extension in 2026 remains one of the most effective ways to upgrade your home in London. The key is working with an experienced construction company that manages design, build and finishing seamlessly.

Planning properly from day one will save you time, stress and unexpected costs.