First-Floor Extensions Over a Single Storey

What you get

A first-floor extension suits you if your house already has a single-storey extension and you need more space upstairs. You get a structural check of the existing walls and foundations, a design that ties the new floor into the house, the approvals your nation needs, building regulations approval and the build, managed by us and carried out by vetted teams working under S&K.

It suits you if…

You already have a single-storey rear or side extension with a flat or low roof
You need a bedroom, bathroom or study upstairs but do not want to lose garden
Your existing extension was built with building control approval and sound foundations
You want to replace an old flat roof at the same time

Line drawing: existing house in outline, new work in brick, blue dashed line at first-floor level.

Design points that matter

Can the existing structure take it?

Many single-storey extensions were built with foundations, walls and lintels sized for one storey. We find out what is there with trial holes and, where available, the original building control drawings. The engineer then decides whether to reuse, strengthen or rebuild parts of the structure.

Replacing the flat roof

The existing roof usually comes off and is replaced by a new first-floor structure. The room below is exposed for a short time, so we plan temporary weatherproofing and sequence the work to keep the kitchen or living room usable for as long as possible.

Tying into the house

The new floor and walls must line up with the existing first floor, often through a new opening in the rear wall above the old one. Floor levels, the position of upstairs windows and the existing roof all affect where the opening can go, so we survey both floors carefully.

Escape and fire safety

A new first-floor bedroom needs an escape window with a clear opening of at least 0.33 m², at least 450 mm wide and 450 mm high, with the bottom no more than 1,100 mm above the floor, unless there is a protected route to the stairs.

Matching the roofline

A first-floor extension often changes how the back or side of the house looks more than the original single storey did. Matching the main roof pitch and materials, and keeping the ridge below the main ridge, usually gives the best result and is expected for permitted development in England.

Planning and approvals by nation

England

In England, adding a first floor turns a single-storey extension into one of more than one storey. To be permitted development, the extension as a whole must go no more than 3 m beyond the rear wall of the original house and stay at least 7 m from the rear boundary, with matching materials and roof pitch. Over a side extension it always needs planning permission, costing £548 (or £575 for applications made from 8 December 2026).

Approval in England: Householder planning application: £548 (or £575 for applications made from 8 December 2026). Lawful development certificate: £274 (about £288 for applications from 8 December 2026). Councils should decide householder applications within 8 weeks of validation.

Building regulations: Building regulations approval (full plans recommended) from the council or a registered building control approver, with a completion certificate at the end.

Wales

In Wales, a first floor over a rear extension can be permitted development if it goes no more than 3 m beyond the original rear wall and stays at least 10.5 m from the rear boundary, with the ground floor within 4 m. Over a side extension, the 10.5 m, 1 m set-back and 50% width tests apply. Otherwise a householder application costs £585.

Approval in Wales: Householder planning application: £585. Councils normally decide householder applications within 8 weeks.

Building regulations: Building regulations approval under the Welsh Approved Documents, which differ from England's in places (notably energy efficiency).

Scotland

In Scotland, permitted development for extensions of more than one storey (Class 1B) needs at least 10 m to every boundary and no part higher than the house. Most first-floor extensions cannot meet the 10 m test, so plan for a householder application (£371, decided within two months) and a building warrant before work starts.

Approval in Scotland: Householder planning application: £371. Councils have two months to decide a local planning application.

Building standards: A building warrant from the council's building standards service before work starts, and a completion certificate at the end. Scotland uses its own Technical Handbooks.

Northern Ireland

In Northern Ireland, a first floor over a rear extension can be permitted development if the extension goes no more than 3 m beyond the original rear wall and stays at least 7 m from the rear boundary, with height, materials and roof pitch conditions. Over a side extension, or beyond these limits, a planning application costs £347 per dwelling.

Approval in Northern Ireland: Planning application for works to a house: £347. Councils aim to decide local applications within an average of 15 weeks; many take longer.

Building control: Building control approval from the local council under the Building Regulations (Northern Ireland), using the Technical Booklets.

Guide price

As a guide, a first-floor extension costs £25,000–£36,500 including VAT at 20% for 4 m × 3.5 m of new first-floor space (14 m²), based on West Midlands prices and our Standard spec; London and the South East are higher.

Guide prices by region
Region	Per m² of new floor	Typical project
London	£2,160–£3,120 per m²	£30,000–£43,500
South East England	£1,980–£2,860 per m²	£27,500–£40,000
East of England	£1,850–£2,680 per m²	£26,000–£37,500
South West England	£1,850–£2,680 per m²	£26,000–£37,500
West Midlands	£1,800–£2,600 per m²	£25,000–£36,500
East Midlands	£1,760–£2,550 per m²	£24,500–£35,500
North West England	£1,710–£2,470 per m²	£24,000–£34,500
Yorkshire and the Humber	£1,660–£2,390 per m²	£23,000–£33,500
North East England	£1,620–£2,340 per m²	£22,500–£33,000
Wales	£1,660–£2,390 per m²	£23,000–£33,500
Scotland	£1,690–£2,440 per m²	£23,500–£34,000
Northern Ireland	£1,580–£2,290 per m²	£22,000–£32,000

Typical project: 4 m × 3.5 m of new first-floor space (14 m²). Including VAT at 20%, Standard spec. Essential spec is about 8% less, Premium about 20% more. See all prices and fees or work out your own guide price.

What's included

Free site visit and survey of both floors
Trial holes and a check of the existing structure
Design that ties the new floor into the house
Permitted development check or planning application
Structural engineering for walls, foundations and floor
Building regulations drawings and approval
Temporary weatherproofing while the roof is replaced
The build, carried out by vetted teams working under S&K

How it works

Free site visit
We look at your house, the boundaries, access and drains, and talk through what you need the new space to do.
Design and approval
Measured survey and drawings, then planning permission or a lawful development certificate, depending on the rules where you live.
Engineering and fixed price
Structural calculations and building regulations approval, then a fixed build price in writing.
Build and sign-off
Vetted teams working under S&K build it, and we arrange the completion certificate.

Questions about first-floor extensions

Can I build on top of my existing single-storey extension?

Often yes, but only after the structure has been checked. Foundations, walls and lintels built for one storey may not carry two. We dig trial holes, look for the original building control approval and ask the structural engineer to confirm what can stay, what needs strengthening and what must be rebuilt.

What happens to the room below while the first floor is built?

The old roof is removed, so the room below is exposed for a short period. We fit temporary weather protection, plan the work so the roof comes off only when the new structure is ready, and agree with you which rooms will be out of use and for roughly how long.

Does a first-floor extension need building regulations approval?

Yes. It is structural work that creates new habitable space, so it needs approval: building regulations in England, Wales and Northern Ireland, and a building warrant in Scotland. In England that covers structure, fire escape, insulation to U-values such as 0.18 W/m²K for walls and 0.15 for roofs, ventilation, electrics and drainage.

Will the first-floor extension match my house?

It should, and in England matching materials and roof pitch is a condition of permitted development. We match brick, tile, mortar colour and window style where we can, and tell you at the design stage if an exact brick match is not available, so you can choose between the closest match and render or cladding.

Is a first-floor extension cheaper than building two storeys from scratch?

Usually, because the ground floor already exists. Our guide is £25,000–£36,500 including VAT at 20% for 4 m × 3.5 m of new first-floor space (14 m²), at West Midlands prices on our Standard spec. If the foundations or walls below need rebuilding, the saving shrinks, which is why we check the structure first.

Related services

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A two-storey rear extension is a new two-floor addition built onto the back of your house, usually giving a larger kitchen or living space on the ground floor and one or two bedrooms or a bathroom above.

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An extension over a garage is a new first-floor room built on top of an existing attached garage, usually a bedroom, en-suite or study.

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Structural engineering for a two-storey extension involves calculating how the new floors, walls and roof carry their loads down to the ground, and designing the beams, lintels, foundations and ties into the existing house to suit.

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