First-floor extensions in County Antrim

County Antrim at a glance

Nation: Northern Ireland
Price region: Northern Ireland
Postcode areas: BT
Towns covered: 15 listed below, plus surrounding villages
Approval route: It may be permitted development if it goes no more than 3 m beyond the original rear wall and stays 7 m from the rear boundary.
Site visits: Free, no obligation

What a first-floor extension involves

A first-floor extension suits you if your house already has a single-storey extension and you need more space upstairs. You get a structural check of the existing walls and foundations, a design that ties the new floor into the house, the approvals your nation needs, building regulations approval and the build, managed by us and carried out by vetted teams working under S&K.

Across County Antrim we cover Belfast (BT1), Lisburn (BT10), Newtownabbey (BT36), Castlereagh (BT4), Carrickfergus (BT38) and Ballymena (BT42), plus the smaller places around them.

Guide price in Northern Ireland

In Northern Ireland, which includes County Antrim, we'd expect a first-floor extension to cost around £22,000–£32,000 for 4 m × 3.5 m of new first-floor space (14 m²), or £1,580–£2,290 per m² including VAT. Keep a contingency of around 10% for things nobody can see until the ground is opened up.

Spec	Per m²	Typical project
Essential	£1,460–£2,100 per m²	£20,500–£29,500
Standard	£1,580–£2,290 per m²	£22,000–£32,000
Premium	£1,900–£2,750 per m²	£26,500–£38,500

Rules in Northern Ireland

In Northern Ireland, a first floor over a rear extension can be permitted development if the extension goes no more than 3 m beyond the original rear wall and stays at least 7 m from the rear boundary, with height, materials and roof pitch conditions. Over a side extension, or beyond these limits, a planning application costs £347 per dwelling.

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.

There is no Party Wall Act in Northern Ireland. Shared walls and boundaries are a matter of title and common law, so we agree access and record conditions with neighbours before work starts.

Getting the design right

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.

Can the existing structure take it?

Many single-storey extensions were built with foundations, walls and lintels sized for one storey. We find out what is there with trial holes and, where available, the original building control drawings. The engineer then decides whether to reuse, strengthen or rebuild parts of the structure.

How we work in County Antrim

Visit and first ideas
One of our team visits your house in County Antrim, checks the walls, roof and drains, and sketches out options with you.
Drawings, then consent
We draw the scheme, check it against the rules in Northern Ireland, and take it through the right consent with the council.
Technical design
We prepare the building regulations drawings, structural calculations and specification so the build price can be fixed.
Build
Foundations, walls, floor, roof, windows, then first and second fix. We break through into the house as late as we can.

Towns we cover in County Antrim

First-floor extensions in County Antrim: questions

How much does a first-floor extension cost in County Antrim?

In Northern Ireland our guide is £1,580–£2,290 per m² including VAT, so about £22,000–£32,000 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 County Antrim?

County Antrim is in Northern Ireland. It may be permitted development if it goes no more than 3 m beyond the original rear wall and stays 7 m from the rear boundary.

Neighbouring areas

All extension services in County Antrim · About first-floor extensions