Two-storey extensions for semi-detached houses in Downpatrick

Downpatrick: the key facts for this kind of project.

Postcode districts: BT30
County / area: County Down, Northern Ireland
Price region: Northern Ireland
Guide price: £49,500–£66,500 (£1,760–£2,380 per m²)
Approval route: Depends on position: a two-storey rear extension may be permitted development (3 m deep, 7 m from the rear boundary); most others need planning permission.
Nearest towns: Ballynahinch (8.9 miles), Newcastle (10.4 miles), Carryduff (14.8 miles)
Distance to Bangor (County Down): about 23.1 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.

Around Downpatrick we also work in Ballynahinch, Newcastle and Carryduff, and across the rest of County Down. Postcode district in and near Downpatrick: BT30.

What it costs in Downpatrick

A guide price for Downpatrick: around £49,500–£66,500 for a 4 m × 3.5 m footprint on two floors (28 m² of new floor space). That works out at £1,760–£2,380 per m² of new floor space, including VAT at 20%. We'll give you a written design-stage price first, then a fixed build price once the drawings are agreed.

Spec	Per m²	A 4 m × 3.5 m footprint on two floors (28 m² of new floor space)
Essential	£1,620–£2,190 per m²	£45,500–£61,000
Standard	£1,760–£2,380 per m²	£49,500–£66,500
Premium	£2,110–£2,850 per m²	£59,000–£80,000

Planning and building control in Downpatrick

In Northern Ireland, a two-storey rear extension on a semi can be permitted development if it goes no more than 3 m beyond the original rear wall and stays 7 m from the rear boundary, with height, materials and roof pitch conditions. Upper side windows within 15 m of a boundary must be obscure and non-opening. A side extension normally needs planning, costing £347.

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.

Design points for Downpatrick 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

Understand the brief
At your home in Downpatrick we agree what you need: bedroom count, bathroom, kitchen space, and how much garden you want to keep.
Choose the approval route
Some designs fit permitted development, most need a householder application. We decide which applies in Northern Ireland before drawing in detail.
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.
On site
One project lead runs the job, keeps you updated and arranges inspections at each stage until completion.

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

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

Downpatrick is in Northern Ireland. Depends on position: a two-storey rear extension may be permitted development (3 m deep, 7 m from the rear boundary); most others need planning permission.

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

Our guide for Northern Ireland, which covers Downpatrick, is £1,760–£2,380 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 £49,500–£66,500. Design and approval fees are extra, and we fix the build price in writing once the design is agreed.

Who handles building control for a project in Downpatrick?

We handle it as part of the project. What's needed in Northern Ireland: building control approval from the local council under the Building Regulations (Northern Ireland), using the Technical Booklets. We prepare the drawings and the engineer's calculations, book the inspections and pass you the completion paperwork at the end.

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 Downpatrick? We work across the area, including:

Other extensions in Downpatrick

All two-storey extension services in Downpatrick · Two-storey extensions for semi-detached houses across County Down

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