Part single, part two-storey extensions in Cardiff

The basics for Cardiff at a glance.

Postcode districts: CF10, CF11 and CF24
County / area: Cardiff and the Vale of Glamorgan, Wales
Price region: Wales
Guide price: £66,000–£91,000 (£1,840–£2,530 per m²)
Approval route: It may be permitted development if the ground floor goes no more than 4 m and the first floor no more than 3 m beyond the original rear wall, with the first floor 10.5 m from the rear boundary.
Nearest towns: Llandaff (1.9 miles), Whitchurch (Cardiff and the Vale of Glamorgan) (2.8 miles), Penarth (2.9 miles)
First visit: Free, no obligation

About a part single, part two-storey extension

A part single, part two-storey extension is an extension where the ground floor is wider or deeper than the floor above, so only part of it rises to two storeys. It usually gives a large kitchen or living area with roof lights below and one bedroom or bathroom above. It is often a good way to add space while limiting the effect on neighbours.

Around Cardiff we also work in Llandaff, Whitchurch (Cardiff and the Vale of Glamorgan) and Penarth, and across the rest of Cardiff and the Vale of Glamorgan. Postcode districts in and near Cardiff: CF10, CF11 and CF24.

What it costs in Cardiff

A guide price for Cardiff: around £66,000–£91,000 for a 6 m × 4 m ground floor and a first floor over half of it (36 m² in total). That works out at £1,840–£2,530 per m² of new floor space, including VAT at 20%. The fastest way to a firmer number is a free site visit, after which we price the design stage in writing.

Spec	Per m²	A 6 m × 4 m ground floor and a first floor over half of it (36 m² in total)
Essential	£1,690–£2,330 per m²	£61,000–£84,000
Standard	£1,840–£2,530 per m²	£66,000–£91,000
Premium	£2,210–£3,040 per m²	£79,500–£109,500

Planning and building regulations in Cardiff

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).

Design points for Cardiff homes

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.

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.

How we'd run your project

Measure and listen
A visit to Cardiff to understand how you live in the house now and what the extra rooms need to do, plus a first look at the structure.
Plans, then approval
Measured survey, design, then the right approval route for your house. We explain each step before we take it.
Structure and building control
A structural engineer sizes the foundations and steels, and we get building regulations approval before work starts.
Build and hand over
We manage the build from groundworks to decoration-ready finish and hand over with the completion paperwork.

Questions about part single, part two-storey extensions in Cardiff

Which postcodes do you cover around Cardiff?

We cover Cardiff (CF10, CF11 and CF24) and nearby places including Llandaff, Whitchurch (Cardiff and the Vale of Glamorgan) and Penarth, plus the rest of Cardiff and the Vale of Glamorgan. The first site visit is free.

Do I need planning permission for a part single, part two-storey extension in Cardiff?

Cardiff is in Wales, so Wales planning rules apply. It may be permitted development if the ground floor goes no more than 4 m and the first floor no more than 3 m beyond the original rear wall, with the first floor 10.5 m from the rear boundary. If in doubt, a lawful development certificate or a planning application gives you certainty before you build.

How much does a part single, part two-storey extension cost in Cardiff?

Our guide for Wales, which covers Cardiff, is £1,840–£2,530 per m² including VAT. A typical project with a 6 m × 4 m ground floor and a first floor over half of it (36 m² in total) comes to about £66,000–£91,000. Design and approval fees are extra, and we fix the build price in writing once the design is agreed.

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 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.

Nearby

We also cover places near Cardiff, including:

Other extensions in Cardiff

All two-storey extension services in Cardiff · Part single, part two-storey extensions across Cardiff and the Vale of Glamorgan

Want a figure first? Try our guide price tool, then book a free visit to firm it up.