Double Storey Extension Cost: UK Prices per m² and Size

Double storey extension cost per m²

Checkatrade’s double storey extension cost guide prices a double storey extension at £1,650 to £3,000 per m², excluding VAT. The rate is split by the quality of the build.

Quality	Range per m²	Average per m²
Standard	£1,650 to £2,150	£1,900
Good	£1,830 to £2,350	£2,090
Excellent	£2,150 to £3,000	£2,575

A double storey extension typically costs 50% to 70% more than a single storey extension, not double, Checkatrade’s guide says. Some costs, such as the foundations, site preparation and scaffolding, are shared by the two storeys.

Double storey side extension cost

A 5m x 4m double storey side, front or wraparound extension costs from around £66,000, excluding VAT, on Checkatrade’s figures. Side extensions are popular on detached and semi-detached homes, the same guide says.

Rear extensions and building over a garage

Rear double storey extensions are the most popular, though they can sharply reduce your garden space, Checkatrade’s guide says. Building on top of an existing garage needs a structural survey first, to confirm the garage can take the weight.

Two storey extension cost for 60m²

Checkatrade’s house extension cost guide prices a two-storey extension based on 60m² at £108,000 to £180,000, with an average of £144,000, excluding VAT.

Checkatrade’s double storey guide prices three parts of a 60m² double storey build, 30m² per floor, at an average of £6,000 for the foundations, £36,000 for the framing and £14,000 for the roofing, excluding VAT. Each is one part of the work, not a price for the whole extension.

Extra costs for fees, a kitchen or a bathroom

Checkatrade’s guide lists these extra costs, excluding VAT. The kitchen and bathroom rows are what each adds to the cost of the extension.

Extra cost	Range	Average
Extension planning drawings	£500 to £1,500	£950
Surveyor’s fees	£500 to £2,000	£1,250
Party wall agreement	£900 to £2,700	£1,000
Bathroom	£2,500 to £11,000	£6,750
Kitchen	£10,000 to £25,000	£15,000

A kitchen or bathroom costs more than a bedroom because of the extra electrics, plumbing and finish, Checkatrade’s guide says. An architect is not a legal requirement, and if you use one, the fees run at 5% to 12% of the whole construction cost, on Checkatrade’s figures.

The drawings, the surveyor and the party wall agreement are fees rather than building work, and the agreement costs roughly £1,000 per neighbour on Checkatrade’s figures.

What affects the cost of a double storey extension

Checkatrade’s guide names size, quality and finish, such as plastering and painting, among the factors in the price. Groundworks, scaffolding for the work at height, site access, drainage for a kitchen or bathroom, and electrics fitted and inspected by a qualified electrician all move the price too. Structural changes may also affect your home insurance premium.

For labour, an experienced builder charges around £50 an hour and a self-employed builder around £40, with plumbers and electricians at around £50, on Checkatrade’s figures. Ask every builder to name the groundworks, scaffolding, drainage and electrics in the quote, so you can see what a cheaper price leaves out.

Among the Nottingham builders compared for extensions, D&M Building Services in Long Eaton builds single and double-storey extensions with an in-house construction team, works to detailed quotations with a defined scope and staged payments, and is a Federation of Master Builders member.

Planning permission for a double storey extension

Permitted development rights let you extend a house without applying for planning permission in many circumstances, but every side extension of more than one storey needs householder planning permission, the Planning Portal says. On Article 2(3) land, every rear extension of more than one storey needs it too, and that land includes conservation areas and National Parks, the Planning Portal’s guidance note says.

Elsewhere, a double storey rear extension has to meet the conditions for more than one storey, and the limits that apply to every extension, to be built without an application. Go beyond any of them and it is likely that you will need to apply for householder planning permission, the Planning Portal says.

Party wall notice for a double storey extension

The Party Wall etc. Act 1996 covers work to an existing party wall, building a new wall at or astride the boundary, and excavating within 3 metres of a neighbour’s building to below its foundations, or within 6 metres where the dig meets a line drawn down at 45 degrees from the bottom of those foundations, among other work, the GOV.UK explanatory booklet says.

You have to give notice at least 2 months before work to an existing party wall, and at least 1 month before building a new wall at the boundary or excavating, GOV.UK’s booklet says. If your neighbour has done nothing 14 days after the notice is served, a dispute is deemed to have arisen, and one agreed surveyor, or one surveyor each, draws up an award. Where the work is solely for your benefit, you usually pay all the costs of the award, including your neighbour’s surveyor, on the booklet’s guidance.

How long a double storey extension takes

Most double storey extensions take 8 to 14 weeks of construction, Checkatrade’s guide says. Before that, the designs, drawings, planning permission and structural planning take from 2 weeks to several months.