Part Single, Part Two-Storey Extensions

What you get

A part single, part two-storey extension suits you if you want a generous ground floor but only need one extra room upstairs, or if a full two-storey design would affect a neighbour too much. You get design options that test where the two-storey part works best, the approvals your nation needs, structural design, building regulations approval and the build, carried out by vetted teams working under S&K.

It suits you if…

You want a large open-plan kitchen but only one more bedroom or bathroom upstairs
A full-width two-storey extension would take too much light from a neighbour
You want roof lights over the kitchen as well as space above
You want to control cost by keeping part of the extension single storey

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

Design points that matter

Where to put the two-storey part

The two-storey section usually works best away from the closest neighbour, with the single-storey part on that boundary side. We test a few positions against neighbours' windows, the upstairs layout and the roof shape before choosing the one to take forward.

The roof junction

Where the single-storey roof meets the two-storey wall you need a sound weathering detail, usually a lead or similar flashing, and enough height for roof lights or a lantern. We set the roof falls and the upstairs window sill heights together so they do not clash.

Light into the kitchen

The single-storey part is the place for roof lights or a lantern, which bring daylight deep into the plan. In England, glazing is normally limited to 25% of the new floor area plus any windows removed, unless calculations show the house still performs, so we plan the balance early.

Structure at the step

The first-floor wall above often sits over the new open-plan space, so it needs a steel beam or frame to carry it. Loads come down at different points than in a full two-storey design, and the engineer sizes the foundations to suit each part.

Planning and approvals by nation

England

In England, the extension can only be permitted development if it goes no more than 3 m beyond the original rear wall and stays at least 7 m from the rear boundary, with matching materials and roof pitch. We check the whole design against the rules, including the 50% garden coverage limit. If any part fails, or extends beyond a side wall on two floors, you need 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 part two-storey rear extension can be permitted development if the first floor goes no more than 3 m and the ground floor no more than 4 m beyond the original rear wall, with the first floor at least 10.5 m from the rear boundary. Conservation areas and other protected areas are excluded. If the design falls outside these limits, 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, because part of the extension is more than one storey, it is assessed under Class 1B, which needs at least 10 m to every boundary for permitted development. On most plots that is not possible, so the extension needs a householder application (£371, decided within two months). A building warrant is also needed before any 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, the extension can be permitted development if it 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 met. Specified protected areas are excluded. If the design goes further, a planning application costs £347 per dwelling, and building control approval is needed either way.

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 part single, part two-storey extension costs £72,000–£99,000 including VAT at 20% for a 6 m × 4 m ground floor and a first floor over half of it (36 m² in total), 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	£2,400–£3,300 per m²	£86,500–£119,000
South East England	£2,200–£3,030 per m²	£79,000–£109,000
East of England	£2,060–£2,830 per m²	£74,000–£102,000
South West England	£2,060–£2,830 per m²	£74,000–£102,000
West Midlands	£2,000–£2,750 per m²	£72,000–£99,000
East Midlands	£1,960–£2,700 per m²	£70,500–£97,000
North West England	£1,900–£2,610 per m²	£68,500–£94,000
Yorkshire and the Humber	£1,840–£2,530 per m²	£66,000–£91,000
North East England	£1,800–£2,480 per m²	£65,000–£89,000
Wales	£1,840–£2,530 per m²	£66,000–£91,000
Scotland	£1,880–£2,580 per m²	£67,500–£93,000
Northern Ireland	£1,760–£2,420 per m²	£63,500–£87,000

Typical project: a 6 m × 4 m ground floor and a first floor over half of it (36 m² in total). 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
Measured survey, priced in writing with the design stage
Design options testing where the two-storey part goes
Permitted development check or planning application
Structural design for the step and openings
Roof light and glazing design within Part L
Building regulations drawings and approval
Party wall notices in England and Wales where needed
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 part single, part two-storey extensions

Is a part single, part two-storey extension cheaper than a full two-storey one?

For the same ground floor it costs less in total, because there is less first floor to build. Per m² it is similar, since the roof junction and steelwork add cost. Our guide for a typical example, a 6 m × 4 m ground floor and a first floor over half of it (36 m² in total), is £72,000–£99,000 including VAT at 20% at West Midlands prices on our Standard spec.

Can the single-storey part be deeper than the two-storey part?

With planning permission, yes, and that is often the point of the design: the ground floor projects further into the garden while the first floor stays back from neighbours. Under permitted development, the depth limits for extensions of more than one storey can apply to the whole extension, so we confirm which route applies before drawings are finalised.

Which side should the two-storey part go on?

Usually the side furthest from the nearest neighbour's windows, so the single-storey part sits nearest them and keeps their light. Other factors are where the stairs land upstairs, which room needs the extra space and where drains run. We sketch options at the design stage and explain which is most likely to be approved.

Can I add roof lights to the single-storey part?

Yes. Roof lights or a lantern over the single-storey part are one of the main reasons to choose this design, as they light the middle of a deep kitchen. In England they count towards the Part L glazing allowance, normally 25% of the new floor area plus windows removed, so we balance them with the rear doors.

Will I need a party wall notice for a part two-storey extension?

In England and Wales you may, if the extension is built on the boundary line, cuts into a shared wall or needs foundations dug within 3 m of a neighbour's building and deeper than theirs. The single-storey part often sits on the boundary, so notices are common. Scotland and Northern Ireland have no Party Wall Act.

Related services

Two-storey rear extensions

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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Kitchen extensions with bedrooms above

A kitchen extension with bedrooms above is a two-storey extension designed so the ground floor becomes a larger, usually open-plan kitchen and dining space, with one or two bedrooms or a bathroom on the first floor.

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First-floor extensions

A first-floor extension is a new upstairs room built on top of an existing single-storey part of your house, such as a rear kitchen extension, a utility or a side annexe.