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2 storey prefab extensions

A 2 storey prefab extension gives you a room downstairs and a bedroom or bathroom above it, with the frame made in a factory. Budget £1,700 to £2,900 per m² of total floor area including VAT. A 3 m by 5 m footprint over two floors is 30 m², so £51,000 to £87,000.

Going up is cheaper per m² than going out, because one set of foundations and one roof serve both floors.

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Two storeys, built and finished

£1,700 to £2,900 per m²

Per m² across both floors, including VAT.

Two storeys make sense when

  • You need a bedroom or bathroom as much as a bigger kitchen.
  • The garden is short and every metre of depth counts.
  • The house is detached, or the neighbours are well away from the side you are building on.
  • You plan to stay long enough for the extra floor to pay its way.
  • You can live with scaffolding for two to three months.

2 storey prefab extension prices

WhatTypical price including VAT
Built and finished, per m²Total floor area over both storeys£1,700 to £2,900 per m²
30 m² in total3 m by 5 m footprint, two floors£51,000 to £87,000
48 m² in total4 m by 6 m footprint, two floors£81,500 to £139,000
2 storey kit, panels onlyPer m² of floor, nothing fitted£1,200 to £1,700 per m²
Structural engineer, two storeysMore work than a single storey: two openings and wind loads£800 to £1,800

The per m² range is for two-storey extensions of every build method; nobody publishes a separate national figure for prefab. Including VAT; London and the South East usually cost 20 to 40% more. Treat it as a starting point. A guide, not a quote. Try your own size in the calculator

Planning is the first hurdle

English permitted development rules allow a two-storey extension only where it reaches no more than 3 m past the original back wall and leaves 7 m or more between itself and the rear boundary. The roof pitch must match the house as far as it can, and any upper-floor window facing sideways has to be in obscured glass. These limits are in the government technical guidance.

Plenty of semis and terraces fail the 7 m test or want more than 3 m of depth. Then you apply for householder planning permission, which costs £548 in England and normally takes eight weeks. In a conservation area there is no permitted development for two storeys at all. Scotland asks for 10 m to every boundary, which rules out most gardens.

Which prefab methods go to two storeys

Timber frame and structural insulated panels both do it routinely; most new timber houses in Britain are two storeys of exactly this. The ground floor walls go up, a floor cassette lands on top, then the upper walls and the roof.

Stacked modules are possible on a detached house with a clear run for a crane. They are rare on semis, because joining two finished boxes to an old house at two levels leaves little room for error.

Structure and the join to the house

Two things make a two-storey job harder than a single. The foundations carry more load, so the engineer may want deeper footings or a raft. And you break through the house wall twice, once per floor, each opening with its own steel beam or lintel.

Old houses settle and new frames do not, so the junction needs a movement joint, not a rigid tie. Ask to see that detail on the drawings before the panels are ordered.

2 storey kits

Kit makers sell two-storey shells at roughly the single-storey rate, £1,200 to £1,700 per m² of floor. We would think hard before running one yourself. Work at height needs scaffolding designed for the job, the panels are heavier, and a mistake in the first-floor cassette is expensive to put right. A firm that both makes and erects is the safer choice here.

Time on site

The frame for both floors is usually weathertight inside two weeks. Roof covering, cladding, two rooms of services and plaster, and a staircase or landing alteration bring the whole job to eight to twelve weeks. A brick and block build of the same size commonly runs four to six months.

Sources for this page

Not sure your garden passes the 7 m rule? A firm can measure it against the plot before you spend on drawings.

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How a two-storey prefab build runs

  1. Check the plot

    Depth, distance to boundaries and the neighbours' windows decide the planning route.

  2. Engineer and order

    Foundations, both openings and the frame are designed together, then the panels are cut.

  3. Frame in two lifts

    Ground floor, floor cassette, first floor and roof, with scaffolding rising as it goes.

  4. Break through and fit out

    Openings at each level, then services, plaster and joinery upstairs and down.

Two-storey questions

How much does a 2 storey prefab extension cost?

Budget £1,700 to £2,900 per m² of total floor including VAT. For 30 m² over two floors that is £51,000 to £87,000, plus design fees of £2,700 to £5,600.

Are 2 storey prefab extension kits sold in the UK?

Yes. Timber frame and insulated panel makers supply them at about £1,200 to £1,700 per m² of floor for the panels. Most buyers pay the maker or a builder to erect the frame.

Can I add a prefab storey on top of an existing extension?

Sometimes. The existing foundations and walls have to be proved strong enough, which usually means trial holes and an engineer's report. Light timber panels help, since they weigh far less than blockwork.

Will I need a party wall notice?

In England and Wales, probably, if you dig within 3 m of the neighbour's building or build at the boundary. Notices go out one to two months before work starts (GOV.UK).

Does a two-storey extension add more value than a single?

An extra bedroom usually moves a house into a higher price bracket, where a bigger kitchen may not. Ask a local estate agent what an extra bedroom is worth on your street before you fix the budget.

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