What a modular extension price includes
Two quotes for the same room can differ by £20,000 and both be fair, because one includes the base and the kitchen and the other stops at the front door of the module. Here is where the line usually falls.
Usually in the price
- Design of the module to suit your house, within the maker's system
- Structure, insulation, roof covering and rainwater goods
- Windows, external doors and any rooflights shown on the drawings
- Wiring, sockets, lighting and heating inside the room
- Plastered or boarded walls and ceilings, decorated or ready to decorate
- A standard base on ordinary ground
- Delivery, and a one-day lift with crane, operator and crew
- Connecting the room to the house and making good outside
- VAT at the standard rate
Usually not
- Planning drawings, the structural engineer and building control
- The council's planning fee
- Party wall surveyors, where you build near a neighbour's wall
- Kitchen units, worktops and appliances
- Sanitaryware and tiling in a shower room, at many firms
- Piles or deeper foundations for clay, trees or made ground
- Moving a drain, a manhole or a gas meter
- Knocking through and redecorating the room you join onto
- Landscaping, paths and a patio outside the new doors
The extras, priced
Typical national ranges for 2026, VAT included. Expect 20 to 40% more in London and the South East. A guide, not a quote.
| Item | Typical cost |
|---|---|
| Planning drawings For a single-storey extension | £1,000 to £2,500 |
| Structural engineer Drawings and calculations | £600 to £1,200 |
| Building control in England Single storey, 10 to 40 m². Each council sets its own fee | £700 to £1,100 |
| Householder planning application, England | £548 |
| Prior approval, larger rear extension, England | £249 |
| Lawful development certificate, England Optional proof that no permission is needed | £274 |
| Party wall surveyor Per neighbour, one surveyor agreed by both sides | £700 to £1,500 |
| Kitchen fitted inside the extension | £15,000 to £35,000 |
| Shower room or bathroom | £5,500 to £9,000 |
| A second day of crane and crew | about £1,400 |
Add 10 to 15% of the total as a contingency. The calculator does the sums for your size.
The grey areas
A few items move between the two lists from one firm to the next. They are where most of the gap between quotes comes from, so ask about each one by name.
The base
"Foundations included" can mean a slab sized for good ground and nothing more. Ask what happens, and who pays, if the trial holes find clay or roots. Piles are designed by the engineer after a ground investigation and can add several thousand pounds.
The opening into the house
Taking out the back wall needs a steel beam, temporary props and someone to make good the plaster and floor on the house side. Some makers do all of it. Others stop at the outside face of the wall and leave the knock-through to your own builder.
Services
A price might include wiring inside the room but not a new circuit from your consumer unit, or a radiator but not the boiler capacity to run it. If the drains need moving, that is almost never in the figure.
Getting it to the garden
The quoted lift assumes the crane can stand on the drive or the road. A narrow street, overhead cables or a lift over the roof can bring a second day, a bigger machine, or a road closure that the council has to approve.
Where these figures come from
Fit-out costs, crane hire and professional fees are typical national ranges from UK cost guides published during 2026. Planning fees are set nationally for England from 1 April 2026. Building control fees come from council charge schedules for 2026/27. The party wall procedure is described in the government's booklet.
Questions about inclusions
Does a modular extension price include the foundations?
Usually a standard base on ordinary ground: a concrete slab, strip footings or a ring of small piles, depending on the maker. Deeper foundations for clay, trees or a sewer nearby are almost always extra, and are priced once an engineer has seen a ground investigation.
Is the crane in the price?
A one-day lift from the drive or the road normally is. If the module has to go over the house, or the road has to be closed, expect a second day or a bigger crane at about £1,400 a day.
Is planning permission included?
Some firms draw and submit the application for you; the council fee is still yours to pay. In England that is £548 for a householder application or £249 for prior approval on a larger rear extension.
Do I need a separate kitchen fitter?
Not always. Some makers fit the kitchen in the factory, others leave a shell with services capped off. Either way the kitchen is normally a separate line, typically £15,000 to £35,000.
Get a quote for your extension
A guide figure only goes so far. Tell us where the house is and what you want built, and a builder can price the real thing.
Your pick:
This site is a free introduction service, not a builder. We look for a firm that builds modular extensions where you live and tell you its name. Your details go to that firm only once you say yes.
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