Two-storey extensions for semi-detached houses in Gorseinon

Useful details for planning a two-storey extension for a semi-detached house in Gorseinon.

Postcode districts: SA4
County / area: Swansea and Neath Port Talbot, Wales
Price region: Wales
Guide price: £51,500–£69,500 (£1,840–£2,480 per m²)
Approval route: Depends on position: rear (3 m first floor, 10.5 m to the boundary) and some side extensions (10.5 m, 1 m set-back) can be permitted development in Wales.
Nearest towns: Pontarddulais (3.1 miles), Llanelli (5.2 miles), Swansea (5.4 miles)
Distance to Swansea: about 5.4 miles
First visit: Free, no obligation

About a two-storey extension for a semi-detached house

A two-storey extension for a semi-detached house is a two-floor addition to the rear, the open side, or both, designed around the attached neighbour on one side. The rear is often the simplest option and can sometimes be permitted development, while a side extension uses the gap on the detached side. Both need care with the shared party wall and the neighbour's light.

We cover Gorseinon (SA4) and the places around it, including Pontarddulais (3.1 miles), Llanelli (5.2 miles) and Swansea.

What it costs in Gorseinon

Expect a two-storey extension for a semi-detached house in Gorseinon to land somewhere around £51,500–£69,500 for a 4 m × 3.5 m footprint on two floors (28 m² of new floor space). Our Wales guide rate is £1,840–£2,480 per m², including VAT. 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 4 m × 3.5 m footprint on two floors (28 m² of new floor space)
Essential	£1,690–£2,290 per m²	£47,500–£64,000
Standard	£1,840–£2,480 per m²	£51,500–£69,500
Premium	£2,210–£2,980 per m²	£62,000–£83,500

Planning and building regulations in Gorseinon

In Wales, a semi can take a two-storey rear extension under permitted development with the ground floor up to 4 m and the first floor up to 3 m beyond the original rear wall, 10.5 m from the rear boundary. A side extension can also be permitted development if it is 10.5 m from the side boundary, set back 1 m and adds no more than 50% width. Otherwise the fee is £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 Gorseinon homes

Keeping the pair balanced

Semis are designed as matching pairs, and a side extension changes that balance. Setting the side extension back from the front wall and keeping its ridge lower helps the pair still read as a pair, which councils' design guides commonly ask for.

The attached neighbour's light

A two-storey rear extension next to the party wall can take light from the neighbour's rear windows. Councils often use a 45-degree test, so the first floor may need to be shorter than the ground floor or set in from the shared boundary.

How we'd run your project

Start at your house
We come to your home in Gorseinon, measure up, check the ground floor walls and roof structure, and note anything near the boundaries.
Get it approved
Once you're happy with the design we submit it to the council for Gorseinon and deal with any questions from the planning officer.
Structure and building control
A structural engineer sizes the foundations and steels, and we get building regulations approval before work starts.
Build it
The new structure goes up first, then we open into the house, finish inside and get the completion certificate.

Questions about two-storey extensions for semi-detached houses in Gorseinon

Which postcodes do you cover around Gorseinon?

We cover Gorseinon (SA4) and nearby places including Pontarddulais, Llanelli and Swansea, plus the rest of Swansea and Neath Port Talbot. The first site visit is free.

Do I need planning permission for a two-storey extension for a semi-detached house in Gorseinon?

Gorseinon is in Wales, so Wales planning rules apply. Depends on position: rear (3 m first floor, 10.5 m to the boundary) and some side extensions (10.5 m, 1 m set-back) can be permitted development in Wales. If in doubt, a lawful development certificate or a planning application gives you certainty before you build.

How much does a two-storey extension for a semi-detached house cost in Gorseinon?

Our guide for Wales, which covers Gorseinon, is £1,840–£2,480 per m² including VAT. A typical project with a 4 m × 3.5 m footprint on two floors (28 m² of new floor space) comes to about £51,500–£69,500. Design and approval fees are extra, and we fix the build price in writing once the design is agreed.

What if my neighbour has already extended?

That often helps. An existing extension next door can reduce the effect of yours on their light, and may show what the council accepts on your street. It can also mean building alongside their wall, so we check its position, foundations and any windows facing your plot at the survey.

Does a two-storey extension add value to a semi-detached house?

It usually adds value, but the gain is never certain. Nationwide's 2025 research found that 10% more floor area was linked to about 5% higher value, and that adding a double bedroom and bathroom to a three-bed, one-bathroom house could add up to 24%. Every street has a ceiling price, so check local sales first.

Nearby

Close to Gorseinon? We work across the area, including:

Other extensions in Gorseinon

All two-storey extension services in Gorseinon · Two-storey extensions for semi-detached houses across Swansea and Neath Port Talbot

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