Two-storey prefab extension kits

A bedroom above and a bigger kitchen below, with the frame made in a factory and stood up in days. Two floors bring an engineer, usually a planning application and a crane, and the price is worked out for your house rather than read off a list.

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What a finished two-storey extension costs

Typical national range £1,700 to £2,900 per m², counting the floor on both levels and including VAT. Three common footprints worked through:

3 x 3 m on two floors (18 m²): £30,500 to £52,000
4 x 3 m on two floors (24 m²): £41,000 to £69,500
5 x 3 m on two floors (30 m²): £51,000 to £87,000

Expect 20 to 40% more in London and the South East. A guide, not a quote. Use the calculator

Planning: why two floors usually need an application

England's permitted development rules do stretch to a back extension of two storeys, on tight terms. It can project 3 m at most from the original rear wall, and the gap between it and the boundary behind must be 7 m or more. Its materials should resemble the house's, the roof pitch should follow the existing one where practical, and any upstairs window facing sideways needs obscured glass with no opening part lower than 1.7 m above the floor. A side extension taller than one storey always needs permission.

Most gardens behind semis and terraces are not long enough for the 7 m rule once a 3 m extension sits in them, so a householder planning application is the normal route. In England it costs £548 from April 2026. The larger-home prior approval route that lets single-storey kits reach 6 or 8 m does not apply to two storeys at all.

Plan for the application in your timetable. The council normally has eight weeks to decide a householder application, and the factory will not start cutting panels until you have consent and approved building regulations drawings.

In Scotland the rules for enlargements of more than one storey differ again, and a building warrant is needed before any work. Check with your council.

Structure: what changes with a second floor

Foundations. Two floors weigh more than one, and the footings are designed to suit. On clay or near trees they may be deeper or piled, which is the biggest swing in the budget.

The join to the house. The new frame is fixed to the existing back wall at both levels, and openings are cut on each floor. Expect steel beams over the ground floor opening and possibly upstairs too, sized by a structural engineer, whose drawings and calculations for a two-storey job typically cost £800 to £1,800.

Fire safety. A new bedroom upstairs needs a way out in a fire. Building regulations set rules on escape windows and on protecting the stairway, and the inspector checks the whole route from the new room to the front door.

Getting it in. Floor cassettes and roof sections are large. A crane on the road or the drive for a day is normal; a contract lift with a crew to run it is typically about £1,400 for the day. On a terrace you may need the council's permission to stand it on the highway.

Who is in charge. On any building job at a home, the contractor or principal contractor takes on the client's legal duties under the CDM Regulations 2015. If you are organising trades yourself around a kit, agree in writing who runs the site.

Sources

Two-storey kit questions

Can I buy a two-storey extension kit off the shelf?

Not really. Single-storey kits come in set sizes because the loads are simple. A second floor carries a floor, a roof and people above the ground floor walls, and it has to tie into your house, so each one is engineered for the house it joins. The supplier still makes it in a factory; it just prices it from drawings.

Is a two-storey kit cheaper per square metre than a single storey?

Usually, yes. The foundations and the roof are shared by two floors, so the cost per square metre of floor comes out lower than for a single-storey room, even though the total bill is much larger. Stairs, a bathroom and alterations upstairs push it back up.

Will the Party Wall Act apply to me?

It will if your footings go below the level of a neighbour’s foundations and their house stands within 3 m, or if you build on or against the wall you share. Then the 1996 Act requires written notice. Two-storey work on a semi or terrace almost always qualifies.

How long does a two-storey kit take to put up?

The frame itself often goes up in under a week, and a crane is commonly booked for a day to lift the floor and roof sections. The months go on what follows: roofing, first fix, the opening into the house on two levels, plastering and the stairs or landing alterations.

Get a quote for two floors

Postcode, then four tap-to-choose questions on size and timing, then your name and number. Free, and done in a minute or two.

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.