Two-storey extensions for semi-detached houses in Monmouth

Before you start in Monmouth, here's what applies.

Postcode districts: NP25
County / area: Gwent, 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: Chepstow (12 miles), Abergavenny (13 miles), Caldicot (15.7 miles)
Distance to Newport (Gwent): about 19.8 miles
First visit: Free, no obligation

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

This service suits you if you own a semi-detached house and want to add bedrooms and living space without moving. On a semi, the main questions are which side to build on and how to treat the attached neighbour. You get a survey, design options, the approvals your nation needs and the build, carried out by vetted teams working under S&K.

We cover postcode district NP25 in and around Monmouth, along with neighbouring Chepstow, Abergavenny and Caldicot.

What it costs in Monmouth

Most of the cost of a two-storey extension for a semi-detached house is set by its size and specification. For Monmouth and the rest of Wales, our guide is £1,840–£2,480 per m² including VAT, so about £51,500–£69,500 for a 4 m × 3.5 m footprint on two floors (28 m² of new floor space). Keep a contingency of around 10% for things nobody can see until the ground is opened up.

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 Monmouth

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 Monmouth homes

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.

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.

How we'd run your project

Free site visit
We visit your home in Monmouth, look at the structure, access and boundaries, and talk through what you want the new rooms to do.
Drawings and permission
Our designers produce plans and elevations, then we take care of planning or permitted development checks with the council covering Monmouth.
Technical design
We prepare the building regulations drawings, structural calculations and specification so the build price can be fixed.
Build and sign-off
Vetted teams working under S&K build the extension, and we arrange the building control completion certificate at the end.

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

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

Our guide for Wales, which covers Monmouth, 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.

Who handles building regulations for a project in Monmouth?

We handle it as part of the project. What's needed in Wales: building regulations approval under the Welsh Approved Documents, which differ from England's in places (notably energy efficiency). We prepare the drawings and the engineer's calculations, book the inspections and pass you the completion paperwork at the end.

Which postcodes do you cover around Monmouth?

We cover Monmouth (NP25) and nearby places including Chepstow, Abergavenny and Caldicot, plus the rest of Gwent. The first site visit is free.

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.

Will a side extension make my semi look like a terrace?

It can if it fills the whole gap to the boundary at first floor and the neighbour does the same. Many councils ask for a first-floor gap to the side boundary or a set-back for this reason. We design the extension to keep the semi reading as one of a pair.

Nearby

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

Other extensions in Monmouth

All two-storey extension services in Monmouth · Two-storey extensions for semi-detached houses across Gwent

Send us your postcode and a few details. We'll get back to you to arrange a visit and talk through your options.