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A modern single-storey wing with tall sliding glass doors beside a paved terrace and lawn

Prefab Extension

A room made in a factory and fitted to your house in weeks. Three ways to buy one, compared below.

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Three ways to buy a prefab extension, compared

  • Kit

    Panels delivered flat. You book the groundworker, the builder and the trades.

    6 to 12 weeks on site

    £1,200 to £1,700

    per m², kit only

  • Panel-built

    One firm makes the panels, stands them up on your foundations and finishes the room.

    4 to 8 weeks on site

    £2,200 to £2,800

    per m², installed

  • Finished module

    A completed room leaves the factory on a lorry and a crane sets it down.

    2 to 6 weeks on site

    £2,100 to £3,300

    per m², installed

These are typical national ranges for 2026, including VAT. London and the South East usually cost 20 to 40% more. Each is a guide, not a quote. A kit price leaves out foundations, labour and finishes. Brick and block, for scale: 10 to 16 weeks and £1,800 to £3,000 per m². Work out your own figure

Which route fits your house

Access decides more than taste. Where a crane can reach the garden, a finished module is quickest. Where the only way round is a side path, panels go through by hand. A kit saves money only if you can manage the trades.

A mobile crane lifting a large wall panel into place against a blue sky

What a prefab extension costs

Most single-storey rooms are 15 to 30 m². Built and finished by one firm, 20 m² comes to £44,000 to £56,000 including VAT, with foundations and design fees on top.

The build price sits level with brick and block or a little above it. What you are buying is time, with builders gone in weeks.

Work out the cost for your size

The rules, in three numbers

  • 3 m

    How far a single-storey rear extension may reach behind an attached house in England with no planning application. Detached houses get 4 m.

  • 0.18

    The U-value, in W/m²K, that new extension walls must meet in England. Ask a panel maker for theirs in writing.

  • 8 weeks

    How long a council usually takes to decide a householder planning application.

Building regulations apply to every extension, however it is made. Scotland also asks for a building warrant before work starts.

Where these numbers come from

How the job gets done

  1. Survey and drawings

    The house is measured, the room drawn and the structure calculated.

  2. Factory and footings

    Panels or a module are made under cover while the base is dug and poured.

  3. Delivery

    Panels stand in two to five days. A finished module is lifted into place in one.

  4. Finish and sign-off

    The house wall is opened, services are joined and building control signs it off.

Questions people ask

What is a prefab extension?

An extension whose walls, floor and roof are made in a factory and joined on foundations at your house. Pre built and ready made extensions are the same thing.

Are prefab extensions cheaper than brick?

Not usually. Brick and block is typically £1,800 to £3,000 per m² and panel-built prefab £2,200 to £2,800 per m², so prefab is level or a little dearer. The saving is in weeks on site. See the cost calculator.

Do prefab extensions need planning permission?

They follow the same rules as any extension, and many single-storey rooms in England need no application. Scotland has its own limits.

Do they need foundations?

Yes. A timber shell is lighter than blockwork, but an engineer and building control decide what your ground needs.

Get a quote for your extension

Postcode, four quick questions, then your contact details. It takes about a minute.

We are an introduction service, not a prefab extension builder. We search for a company that takes work near you and give you its name first. Your details stay with us unless you agree to the introduction, and asking is free.

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