Modular house extensions
A volumetric modular extension is a finished room built in a factory, with the walls, roof, windows, wiring and often the plaster and flooring done. It arrives on a lorry and is lifted onto foundations prepared while it was being built.
Installed prices usually run £2,100 to £3,300 per m² before VAT, including groundworks and the crane, with two to six weeks on site.
Typical 25 m² modular extension
£53k to £83k
Installed, single storey, before VAT
Good reasons to go modular
- You want the shortest possible time with builders at the house: often a single day of lifting.
- There is room for a crane, on a drive, the road or a rear lane, and a clear swing over the house.
- A simple, rectangular room: kitchen-diner, garden room, bedroom with shower room or a home office.
- You like knowing the specification and price early, before any ground is dug.
- Factory quality control: walls built square and dry, with tested insulation and airtightness.
- A bungalow or house where a single-storey rear box is the natural shape.
Modular extension prices by finish
| Item | Typical price, ex VAT |
|---|---|
| Standard finish uPVC or basic aluminium glazing, standard flooring and lighting | £2,100 to £2,500 per m² |
| Good finish Aluminium sliders or bifolds, roof light, better floor and joinery | £2,500 to £2,900 per m² |
| Excellent finish Large-format glazing, triple glazing, bespoke cladding and joinery | £2,900 to £3,300 per m² |
| Crane for the lift day Usually included; worth checking. Poor access adds more | £800 to £3,000 |
| 25 m² single storey, all in A 5 x 5 m rear extension, standard to excellent finish | £52,500 to £82,500 |
Where these figures come from
Typical ranges from published UK contractor and builder price guides, read November 2026, before VAT at 20%. Per m² rates include standard groundworks and a one-day crane lift. A guide, not a quote: the price for your house depends on a survey, the ground and the specification. Work out your own figure with the cost calculator.
Not sure a crane can reach? Send us the details and a specialist will look at the access.
Ask for a free quoteWhat arrives on the lorry
Most extension modules are up to about 3.5 m to 4.5 m wide and up to 12 m or so long, because that is what travels on a standard trailer without special movement orders. Larger rooms are made from two or more modules joined on site, with the seam hidden in the ceiling or under a beam.
A good factory finishes as much as possible: external cladding, windows and doors, wiring to the consumer unit, first fix plumbing and often internal plaster and flooring. The less left to do on site, the more of the time saving you keep.
Crane access is the first question
Before you spend on design, check the crane. A mobile crane needs a firm, level spot for its outriggers, close enough to reach over the house to the back garden. Overhead cables, trees and a narrow terrace street all matter. Most lifts take 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 where access is difficult or a road has to be closed.
Where a crane cannot reach, a prefab panel or SIPs system that can be carried through a side gate is usually the answer.
Is modular cheaper?
Not usually by much. Published comparisons for a 25 m² rear extension put a modular build within a few per cent of a traditional quote, because the foundations, drainage, the connection to the house and the inside finishes cost the same. The factory-built shell itself is a little cheaper, and the crane and transport add a little back.
Where modular does save money is time: two to six weeks on site rather than ten to sixteen, so less disruption, fewer weeks of skip and scaffold hire, and a kitchen out of use for less time.
Planning, building control and warranties
The planning limits are the same as for any extension; the configurator on our modular extension page draws them against your size. Building regulations approval is always needed. Ask the maker which elements are certified in the factory and what building control will inspect on site, and get the structural warranty or latent defects cover in writing before you pay a deposit.
From order to finished room
Survey and access check
Measured survey, crane position, ground conditions and the planning route.
Design and approvals
Final drawings, planning or prior approval if needed, building control plans.
Factory and foundations
Modules built over four to six weeks while the base and drainage go in at home.
Lift day and finish
Modules craned in, joined to the house, finished inside and signed off in two to six weeks.
Questions people ask
How much does a modular house extension cost?
Typically £2,100 to £3,300 per m² before VAT, installed, including groundworks and the crane. A 25 m² single-storey extension usually costs £52,500 to £82,500.
How big can a modular extension be?
Single modules are limited by road transport, usually to about 4.5 m wide. Larger rooms join two or more modules on site. Planning limits often set the depth before the factory does.
What if a crane cannot get to my garden?
Then a panel-built, prefab or SIPs extension is usually the better fit: the parts are carried through a side gate and put together by hand or with a small lifting aid.
Can a modular extension be two storeys?
Yes, by stacking modules or using panels. It needs more structural work where it joins the house, and the planning limits for two-storey rear extensions are tighter. See our two-storey prefab extension page.
Do I have to move out?
Rarely. The noisy, messy part at home is mostly the foundations and the day the wall to the house is opened. Many people keep using the house throughout.
Does a modular extension add value?
An extension adds the value of the extra space in your area, however it was built, as long as it has the right approvals, building control completion and a warranty. Keep every certificate for when you sell.
Get prices for a modular 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.
Thanks, your enquiry is in.
- We search for a firm that makes and fits the type of extension you picked, in your part of the country, then email or ring you with its name.
- Your details reach that firm only if you say yes.
- They look at the house and the access, then price the job. No marketing from us, ever.