First-floor extensions in Cardiff and the Vale of Glamorgan

Cardiff and the Vale of Glamorgan at a glance

Nation: Wales
Price region: Wales
Postcode areas: CF
Towns covered: 10 listed below, plus surrounding villages
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.
Site visits: Free, no obligation

What a first-floor extension involves

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. It adds a bedroom, bathroom or study without enlarging the footprint. Whether it works depends mainly on whether the walls and foundations below can carry the extra load.

Across Cardiff and the Vale of Glamorgan we cover Cardiff (CF10), Barry (CF62), Penarth (CF64), Llantwit Major (CF61), Whitchurch (Cardiff and the Vale of Glamorgan) (CF14) and Rhiwbina (CF14), plus the smaller places around them.

Guide price in Wales

Expect a first-floor extension in Cardiff and the Vale of Glamorgan to land somewhere around £23,000–£33,500 for 4 m × 3.5 m of new first-floor space (14 m²). Our Wales guide rate is £1,660–£2,390 per m², including VAT. We'll give you a written design-stage price first, then a fixed build price once the drawings are agreed.

Spec	Per m²	Typical project
Essential	£1,520–£2,200 per m²	£21,500–£31,000
Standard	£1,660–£2,390 per m²	£23,000–£33,500
Premium	£1,990–£2,870 per m²	£28,000–£40,000

Rules in 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).

The Party Wall etc. Act 1996 applies in Wales, so notices may be needed for work on or near a shared wall or boundary.

Getting the design right

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.

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.

How we work in Cardiff and the Vale of Glamorgan

Free site visit
We visit your home in Cardiff and the Vale of Glamorgan, look at the structure, access and boundaries, and talk through what you want the new rooms to do.
Drawings, then consent
We draw the scheme, check it against the rules in Wales, and take it through the right consent with the council.
Price it properly
Once the engineer's calculations are in, we fix the build price in writing, with a clear list of what's included.
Build and hand over
We manage the build from groundworks to decoration-ready finish and hand over with the completion paperwork.

Towns we cover in Cardiff and the Vale of Glamorgan

First-floor extensions in Cardiff and the Vale of Glamorgan: questions

How much does a first-floor extension cost in Cardiff and the Vale of Glamorgan?

In Wales our guide is £1,660–£2,390 per m² including VAT, so about £23,000–£33,500 for 4 m × 3.5 m of new first-floor space (14 m²). Essential and Premium specs move that by roughly 8% down or 20% up.

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.

Will I need planning permission in Cardiff and the Vale of Glamorgan?

Cardiff and the Vale of Glamorgan is in Wales. 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.

Neighbouring areas

All extension services in Cardiff and the Vale of Glamorgan · About first-floor extensions