House extension costs compared by extension type

How the types differ

This is about relative behaviour rather than fixed figures, because ground, access and specification differ on every site. The pages on single storey extensions and kitchen extensions show how those projects are scoped and built.

The fixed costs nobody escapes

Some costs barely move with size, so they weigh proportionally heavier on small extensions: design work, structural calculations, building control, scaffolding, welfare and site setup, and the connection points into the existing house.

This is why a very small extension can feel poor value per square metre, and why stretching a design slightly sometimes costs less per metre than people expect.

Where the existing house bites

Extensions interact with what is already there, and older properties bring their own conditions.

• Shallow or unknown existing foundations that need matching or underpinning
• Drains running through the proposed footprint
• Solid walls with no cavity, changing how the junction and insulation are detailed
• Roof lines that do not lend themselves to a clean tie in
• Existing heating capacity that cannot serve the extra rooms

Comparing quotes properly

Three quotes are only useful if they price the same job. Give every builder the same drawings, the same specification and the same list of what you expect included, then compare line by line.

A materially cheaper quote usually means something has been excluded or assumed. Find out which before deciding, because those items reappear later as extras.

Budgeting the whole project

Build cost is one part of it. Add professional fees, approvals, kitchens or bathrooms, flooring, decoration, and the cost of living around the works, whether that is takeaways for six weeks or renting somewhere.

A sensible contingency is not pessimism. On extensions, the unknowns are underground and inside walls, and they are only revealed once work starts.