Small prefab extension kits

A kit under 15 m² is the quickest extension most houses will ever get: no planning application behind most homes, a slab poured in a couple of days and a shell that goes up in one. Here is what the small sizes cost and what fits in them.

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Four small footprints, priced

2.5 x 2.5 m
A downstairs WC and coats, or a boot room
Kit, shell only
£7,500 to £11,500
Finished, with VAT
£13,000 to £20,500
3 x 2.5 m
A utility room with space to dry washing
Kit, shell only
£9,000 to £13,500
Finished, with VAT
£16,000 to £25,000
3 x 3 m
A home office for one, or a snug
Kit, shell only
£11,000 to £16,000
Finished, with VAT
£19,000 to £29,500
4 x 3 m
A kitchen pushed out to take a dining table
Kit, shell only
£14,500 to £21,500
Finished, with VAT
£25,000 to £39,500

Typical national ranges for 2026, worked out per square metre: a guide, not a quote. Expect small kits near the upper end of each range. Price another size

Why small kits make sense

Most of the time saved by building from a kit is saved on the walls and roof. On a small room those are a bigger share of the job than on a large one, so the factory route pays off well. The panels for a 3 x 3 m room fit on one lorry and two fitters can stand them in a day.

Small rooms also dodge the expensive extras. A 3 m deep extension rarely needs a steel beam bigger than the one over the new opening, the foundations are short, and the roof can be a simple flat one with a single rooflight.

Where the sums get worse is plumbing. A WC or utility room needs a waste pipe that falls to the drain, and if the nearest manhole is on the other side of the house that one run can cost more than the panels. Check where your drains are before you settle on a room with water in it.

The rules on a small build

Planning. Behind an attached house in England, a one-storey back extension projecting up to 3 m needs no application; a detached house gets 4 m. Overall height tops out at 4 m, and any wall closer than 2 m to the boundary keeps its eaves to 3 m or lower. Count earlier extensions and outbuildings too: together they may not take up more than half the land around the original house. Flats, maisonettes, listed buildings and houses whose permitted development rights have been removed are outside these rules.

Building regulations. Every kit needs approval, small or not. For a simple single-storey room in England many people use a building notice, which lets work start sooner, though the inspector then assesses everything on site rather than from approved plans. The kit supplier's structural calculations and insulation figures are what the inspector will ask for, so make sure they come with the order.

Electrics. Wiring a new room is notifiable work. A registered electrician self-certifies it; if anyone else does it, building control has to inspect.

Scotland and Wales. Scotland runs its own permitted development classes, and no work may begin there until a building warrant is granted. Wales follows rules close to England's, set by the Welsh Government. Check with your council before ordering.

Sources

Small kit questions

What is the smallest prefab extension kit I can buy?

Suppliers will usually draw anything from about 2 x 2 m, but some set a minimum order. Below roughly 6 m² the fixed costs of drawings, engineering and delivery make a kit poor value, and a porch or a small block-built extension may cost less.

Does a small extension need building regulations approval?

Yes. Any extension with solid walls and a roof needs it, however small. The exemptions in the regulations cover a porch, or a mostly glazed conservatory under 30 m² with external-grade doors between it and the house and no fixed heating. A kit room with insulated walls is neither.

Can a small kit sit on my existing patio?

Almost never. A patio is a few centimetres of paving on sand, not a foundation. The kit needs proper footings dug to firm ground, a damp-proof membrane and an insulated floor, so the patio comes up first.

Will a 3 m deep kit block light to my neighbour?

The English rules were written with this in mind, which is why attached houses get only 3 m of projection and eaves close to a shared boundary are capped at 3 m. A flat roof keeps the wall at the boundary low. Talking to the neighbour before the delivery lorry arrives saves a lot of grief.

Get a quote for a small kit

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Prefab Extension Kits does not make kits. We match your footprint with one supplier covering your area, give you its name first, and only send your details when you give the word.