Prefab extension kit prices by size

Set the width and depth of the room you want and see what the kit costs, what the finished room costs and whether planning is likely to come into it. Every figure is a typical national range for 2026: a guide, not a quote.

Kit, shell only: £1,200 to £1,800 per m²
Finished room, including VAT: £2,100 to £3,300 per m²
Both floors of a two-storey build: £1,700 to £2,900 per m²

Price your footprint

Width along the house

4 m

Depth into the garden

3 m

Storeys

Single storeyTwo storey

Show the price for London or the South East

4 x 3 m, 12 m² of floor

The kit£14,500 to £21,500Panels, roof structure, fixings and drawings

The finished room£25,000 to £39,500Foundations, fitting, roof covering, services, decoration and VAT included

Within the 3 m depth that permitted development allows behind any house in England, subject to the height and boundary limits.

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National ranges for 2026; finished figures include VAT. A guide, not a quote.

Seven common sizes

These are the footprints suppliers are asked for most, from a lean utility room to a full-width kitchen. Every kit is cut to order, so a size between two rows costs what its floor area suggests.

Size	Floor	Kit (shell)	Finished room	Room for
3 x 2.5 m	7.5 m²	£9,000 to £13,500	£16,000 to £25,000	Utility and WC, or a garden-facing desk
3 x 3 m	9 m²	£11,000 to £16,000	£19,000 to £29,500	Home office or a snug
4 x 3 m	12 m²	£14,500 to £21,500	£25,000 to £39,500	Dining table for six by new doors
5 x 3 m	15 m²	£18,000 to £27,000	£31,500 to £49,500	Wider dining space across a semi
5 x 4 m	20 m²	£24,000 to £36,000	£42,000 to £66,000	Kitchen with an island
6 x 4 m	24 m²	£29,000 to £43,000	£50,500 to £79,000	Kitchen, table and a sofa
8 x 4 m	32 m²	£38,500 to £57,500	£67,000 to £105,500	Full-width living room on a detached house

Single storey, national, 2026. The kit column prices the shell at £1,200 to £1,800 per m²; the finished column, which includes VAT, uses £2,100 to £3,300 per m². Both rounded to the nearest £500.

What pushes a kit price up or down

Glazing. Big sliding or bifold doors and a roof lantern are the most expensive parts of a small extension. If one quote looks far lower than the rest, check whether the doors are in it.

Roof shape. A flat roof is the simplest and cheapest to make. A lean-to pitch adds rafters and a tiled or slate finish. A pitched roof with a gable is the most work in the factory and on site.

Wall build-up. Kits are made from timber frame panels or structural insulated panels (SIPs). Both can meet the insulation standard in Approved Document L. Thicker walls cost more but leave a warmer room and lower heating bills.

Finish on the outside. Render and timber cladding are often part of the kit. Brick slips cost more. Real brick is laid on site by a bricklayer and sits in the finishing budget, not the kit.

Access. A kit for a terrace with no side access may need to come through the house in smaller pieces, or be lifted over by crane. Either can add to the delivery charge, so tell the supplier at the start.

From kit price to finished price

On every size the finished room costs a little under twice the kit, though the two are not quite like for like: finished prices include VAT, and kit quotes may not. The gap pays for foundations and a slab, assembly, a steel beam over the new opening, roofing, wiring, heating, plaster and floors, plus fees for the engineer and building control. Our guide to finishing a kit breaks that down.

Going up a floor changes the sums. No supplier sells a two-storey kit off a price list, because the frame, the floor between and the foundations all need an engineer's design for your house. The calculator above shows the finished figure only for two storeys, per square metre of floor across both levels. See two-storey kits.

Sources

We base these prices on 2026 UK cost guides, worked out per square metre and rounded by us. They are not offers. Rules and VAT treatment come from GOV.UK.

Questions about kit prices

Why is a kit priced per square metre?

Most of a kit is wall and roof panel, and the amount of panel grows with the floor area. It is a fair way to compare sizes, though very small kits cost a little more per square metre because the drawings, engineering and delivery cost the same whatever the size.

Do the kit prices include windows and doors?

Some kits do and some do not. Glazing is often the largest single item, so ask every supplier to say in writing whether doors, windows and rooflights are in the price before you compare.

Are these prices with VAT?

The finished-room figures include VAT. Kit suppliers quote both ways, so ask whether a kit price includes the 20%. An extension to an existing house is standard rated and a homeowner cannot reclaim it.

Why does living in the South East change the finished price but not the kit?

The kit is made in a factory and priced the same wherever it is delivered, give or take the haulage. The groundwork, fitting and trades are local labour, and that is why finishing in London and the South East usually comes in 20 to 40% above the national range.

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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.