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

How Much Does a House Extension Cost? At a Glance

These are the starting points quoted in this guide for London in 2026. Larger footprints, steelwork, drainage changes and higher specifications cost more, and the sections below explain what moves each figure.

How Much Does a Single Storey Extension Cost?

Single storey rear extensions are the most common type of house extension in London. In 2026, expect straightforward projects to start from around £35,000, rising to £45,000 to £60,000 for larger footprints or higher specification finishes.

The cost per square metre typically ranges from £2,000 to £2,500 for good quality work. This includes foundations, brickwork, roofing, windows, doors, and a basic interior finish. Kitchen fit outs and high-end finishes are extra.

How Much Does a Double Storey Extension Cost?

Double storey extensions usually start from around £65,000 for a modest two-floor addition. Larger double storey extensions can easily run from £85,000 to £120,000 depending on specification and structural complexity.

While double storey extensions cost more upfront, they often provide better value per m² than single storey extensions because foundations and roof costs are spread across two floors.

How Much Does a Wraparound Extension Cost?

Wraparound extensions, combining a side return and rear extension, typically start from £55,000 for a modest L-shaped extension. Larger wraparounds in prime London locations can cost £100,000 to £150,000.

The L-shaped layout created by a wraparound extension is highly popular, often creating 40 to 60 m² of open plan kitchen diner space. This can add £100,000 to £200,000 to a London property's value.

How Much Does a Side Return Extension Cost?

Side return extensions fill in the narrow alleyway alongside Victorian and Edwardian houses. Straightforward projects typically start from around £30,000, with wider spans, steelwork, and drainage changes increasing the cost.

Side returns are cost effective because they use existing walls on two or three sides. However, they often require Party Wall Agreements and drainage alterations, which add to costs.

What Factors Affect Extension Costs?

Key cost factors include size in m², ground conditions, access, specification, and location. Central London usually costs 15 to 25% more than outer boroughs.

Other factors include planning fees (a nationally set fee for householder applications, listed on gov.uk), Building Control fees (£500 to £1,000), Party Wall Agreements (£700 to £1,500 per neighbour if surveyors are needed), and VAT.

Real Extension Cost Examples from London

Islington rear extension (4x3.5m single storey): £38,000, including bifold doors, underfloor heating, and skylights. Completed in 10 weeks.

Wandsworth wraparound extension (L-shaped, 45 m²): £72,000, creating a large kitchen diner with utility room and WC. Completed in 14 weeks.

Hackney double storey extension (3x5m): £68,000, adding a kitchen extension below and a new bedroom above. Completed in 15 weeks.

What Should Your Extension Quote Include?

A good quote lists the work in stages rather than giving one lump sum: groundworks and foundations, structural steel and any beams, brickwork and roofing, windows and doors, electrical and plumbing first fix, plastering, second fix and finishes. When quotes are broken down like this you can compare them line by line and see exactly where one builder is cheaper or more expensive than another.

Ask what is excluded. Kitchens, flooring, decoration, landscaping and light fittings are often left out, and so are professional fees. A fixed price contract should say clearly what is covered and how any changes will be priced before the work is done.

What Costs Do People Forget to Budget For?

Beyond the build itself, most extension projects also involve architectural drawings, a structural engineer's calculations, a planning or lawful development application where one is needed, Building Control fees, and party wall surveyor fees if you share a wall with a neighbour. VAT at 20% applies to most extension work in England, so check whether the figures you are given include it.

It is sensible to keep a contingency of around 10% of the build cost for the unexpected, such as poor ground found when the foundations are dug, or services that need to be moved. If the contingency is not needed, it stays in your pocket.

Next Steps

If you are planning an extension in London or the Home Counties, we can visit, discuss what you want and give you a written, itemised fixed price. Call 020 4569 0526 or use our contact page to book a visit.

Sources and Further Reading

Planning application fees: https://www.gov.uk/guidance/fees-for-planning-applications

Party walls: https://www.gov.uk/party-wall-etc-act-1996-guidance

Planning Portal, extensions: https://www.planningportal.co.uk/permission/common-projects/extensions/planning-permission