Modular Extension: UK Costs, Planning and Prices

Four ways to build an extension, and what each costs

“Modular” covers more than one method. Whole factory-finished boxes need a crane and good access; factory panels and SIPs arrive flat and can go through a side gate. Prices per m² of floor, single storey, before VAT, from published UK contractor and builder price guides, read November 2026.

Volumetric modules

Finished boxes built in a factory, craned in whole

£2,100 to £3,300per m²

2 to 6 wkson site

Factory timber frame panels

Wall, floor and roof panels made off site, assembled in days

£2,200 to £2,800per m²

3 to 8 wkson site

SIPs panels

Insulated sandwich panels cut to your drawings

£2,400 to £3,000per m²

3 to 8 wkson site

Brick and block (for comparison)

Built on site course by course

£1,800 to £2,200per m²

10 to 16 wkson site

Also on this site: two-storey prefab extensions, modular granny annexes and the rules for prefab extensions in Scotland.

Modular extension prices by size

Single-storey rear extensions, installed, from a standard finish at the low end to an excellent one at the high end. Includes standard strip foundations, the crane lift and connection to the house. Before VAT at 20% and before design and council fees.

Floor area	Example footprint	Typical price
12 m²	4 x 3 m	£25,000 to £39,500
20 m²	5 x 4 m	£42,000 to £66,000
25 m²	5 x 5 m	£52,500 to £82,500
30 m²	6 x 5 m	£63,000 to £99,000
40 m²	8 x 5 m	£84,000 to £132,000

Work out your own figure

What moves the price

Glazing. A wall of sliding or bifold doors and a large roof light is the biggest single jump between a standard and an excellent finish.
Ground. Strip foundations cost £1,000 to £3,000; clay or nearby trees can mean a raft at £3,000 to £6,000 or piles from £6,000.
Access. A clear drive keeps the crane small. Lifting over a terrace roof or closing a road costs more.
Where you live. Published guides add 20 to 30% for London and the South East.
Fees. Allow £6,600 to £10,200 for design, a structural engineer and building control.

Where these prices come from

Rates per m² and the crane and foundation figures are typical ranges from published UK contractor and builder price guides, read November 2026, which give £2,100 to £3,300 per m² for a modular single-storey rear extension in 2026, including groundworks and craneage. A guide, not a quote: your price depends on the survey, the ground and the specification.

Planning and building regulations for a modular extension

Planning law does not care how the walls were made; it looks at size, height and position. These are the householder limits in England. Wales has similar rules, and Scotland’s are set out on our Scotland page.

Route	Single-storey rear	Two-storey rear	Who decides, and the fee
Permitted development	Up to 3 m deep (4 m on a detached house), no taller than 4 m, eaves no higher than 3 m within 2 m of a boundary	Up to 3 m deep and at least 7 m from the rear boundary, roof pitch to match	Nobody, though a lawful development certificate (£274) proves it if you sell
Prior approval	3 to 6 m deep (4 to 8 m detached)	Not available	The council notifies neighbours and has 42 days; £249
Planning permission	Anything larger, flats, and most extensions in conservation areas, national parks and on listed buildings	Anything over 3 m deep or nearer than 7 m to the rear boundary	Householder application, usually eight weeks; £548
Building regulations	Always needed: structure, fire spread, insulation, ventilation, drainage and safe glazing. The factory’s certificates help, but the site work and the joint to the house are checked too		Council or registered building control approver; £600 to £1,200
Party wall notice	Foundations within 3 m of a neighbour’s building and deeper than theirs, or work on a shared wall: one or two months’ notice		Your neighbour, or surveyors if they dissent; £1,500 to £3,000 per neighbour

Where our information comes from

MHCLG, Permitted development rights for householders: technical guidance
GOV.UK, Building regulations approval
GOV.UK, The Party Wall etc. Act 1996: explanatory booklet
GOV.UK, Lawful development certificates
Planning fees: council fee schedules for England, 2026. Professional fees: published UK builder price guides, 2026.

How a modular extension gets built: crane day

Weeks of work happen out of sight. The foundations go in while the modules are built in the factory, then everything comes together on one day. A typical timeline for a single-storey volumetric job:

7:00
Road or drive closed if the permit needs it. Crane arrives, outriggers set on spreader mats.
8:00
Lift plan briefed by the appointed person. Everyone not lifting stays out of the swing area.
9:00
First module or panel set lifted over the house onto the prepared base.
11:00
Further modules or the roof cassette set, levelled and fixed down.
14:00
Crane packs down and leaves. Weathering and the joint to the house begin.
Weeks 2 to 6
Opening into the house, services, plastering at the junction, finishes, sign-off.

Weeks on site compared

Modular or panel built
factory 4 to 6 weeks, site 2 to 6

Brick and block
10 to 16 weeks on site

Factory build, groundworks at the same timeOn siteOne square = one week

Questions about modular extensions

How much does a modular extension cost in the UK?

Typical installed prices run from about £2,100 to £3,300 per m² before VAT for a single-storey extension, including groundworks and a one-day crane lift. A 25 m² rear extension usually lands between £53k to £83k. Figures from published UK contractor and builder price guides, read November 2026. Try the cost calculator for your size.

Is a modular extension cheaper than a traditional one?

Not by much. Published comparisons put the build cost within a few per cent either way, because foundations, the connection to the house and the finishes cost the same however the walls are made. The saving is time: two to six weeks on site against ten to sixteen for brick and block, so less rent, disruption and site cost.

Do I need planning permission for a modular extension?

The planning rules treat a modular extension like any other. In England a single-storey rear extension up to 3 m deep (4 m on a detached house) can be permitted development, and up to 6 m (8 m detached) through prior approval. See the government technical guidance and our table of limits below.

Does a modular extension need building regulations approval?

Yes. It is an extension to a dwelling, so it needs building control sign-off for structure, fire, insulation, ventilation and drainage, whatever the factory has certified. Without it you will not have the completion certificate a buyer’s solicitor asks for (GOV.UK).

How long does a modular extension take?

Usually 6 to 10 weeks from order to finish: four to six weeks of factory build while the foundations go in, then 2 to 6 weeks on site. Design, planning and building control approval come before that and often take longer than the build.

Do I need a crane?

For whole modules, yes. Most lifts are a single day, and published UK contractor and builder price guides, read November 2026 put the crane at £800 to £3,000 with £1,500 to £3,000 more for difficult access. Panel and SIPs systems can usually be carried through by hand, which suits terraces with no side access.

Can I get a mortgage or sell a house with a modular extension?

An extension with planning approval where needed, building control completion and a structural warranty is treated like any other. Lenders look closely at whole modular houses, but an extension joined to a conventional house is assessed as part of that house. Keep every certificate in one folder.

Ask for prices on your extension

Three short steps: where the house is, what you want to add, and how to reach you. About two minutes.

This is a free matching service, not a modular extension specialist. Once we have found a firm that builds in your part of the country, we tell you its name, and your details stay with us until you agree.

Step 1 of 3

Where is the house?

Postcode