What is a double-storey extension?
A double-storey extension (also called a two-storey extension) adds rooms on both the ground and first floor in one build. It typically gives a larger kitchen or living space downstairs and an extra bedroom or bathroom above.
How much does a double-storey extension cost in County Antrim?
In County Antrim, a double-storey extension typically costs £1,400–£2,150 per m² of floor area (both floors), including VAT at 20% (standard rate). For a typical 36 m² project that is roughly £50,500–£77,500. That range is adjusted for Northern Ireland, where domestic building work typically costs around 14% below the national average. Design, structural calculations and council fees are extra.
| Size or type | Guide range |
|---|---|
| Smaller project (25 m²) | £35,000–£54,000 |
| Typical project (36 m²) | £50,500–£77,500 |
| Larger project (50 m²) | £70,000–£107,500 |
| Rate per m² | £1,400–£2,150 |
What moves the price most:
- Brick matching and external finishes
- Bathrooms or en-suites on the first floor
- Roof design and how it joins the existing roof
- Scaffolding and access to the rear
Your exact price depends on the survey and drawings. The guide price tool gives a quick range in the meantime.
Do you need planning permission for a double-storey extension in County Antrim?
Here is how the rules work for a double-storey extension in Northern Ireland. In Northern Ireland an extension of more than one storey can be permitted development at the rear if it projects no more than 3 metres and is at least 7 metres from the rear boundary. Two-storey side extensions, and any in a conservation area, need planning permission.
Northern Ireland has no Party Wall Act. Work on shared walls relies on agreement with the neighbour and common law, so we agree the arrangements in writing before work starts.
We check this for your address before design work starts, so there are no surprises later.
What we handle
The work we manage on a double-storey extension:
- Opening up at both levels with steel beams
- Building control sign-off
- Brick or block walls, floors and a roof tied into the house
- Windows, doors and staircase alterations
- Design drawings and planning application
- Electrics, heating, plumbing and plastering
How long does it take?
Allow 14 to 20 weeks on site, after planning and building control approval. Design and approvals come first and typically take 3 to 4 months, depending on the approval route.
Building in County Antrim
Across Northern Ireland, bungalows and chalet bungalows are common, so loft conversions and rear extensions are popular projects.
Towns we cover in County Antrim include Belfast, Lisburn, Newtownabbey, Carrickfergus, Ballymena and Antrim.
How it works
- 01
Visit and measured survey
A free visit to understand the project, check access and take measurements.
- 02
Drawings and written quote
We prepare the design and give you a written, itemised quote with inclusions and exclusions.
- 03
Approvals in place
We handle planning, building control and neighbour notices before work starts.
- 04
Construction and completion
We build, put right any snags and hand over the building control certificate.
Questions about double-storey extensions in County Antrim
How much does a double-storey extension cost in County Antrim?
In County Antrim, a double-storey extension typically costs £1,400–£2,150 per m² of floor area (both floors), including VAT at 20% (standard rate). A typical 36 m² project comes to roughly £50,500–£77,500. Your fixed price depends on the survey and drawings.
Will the first floor need a new staircase?
Not normally. The new rooms are reached from the existing landing, though a doorway or short corridor may need to be formed.
Can I live at home during the work?
Most families stay put. The shell is built first and the house is opened up late in the job, so disruption inside is limited to a few weeks.
Do I need planning permission for a double-storey extension in County Antrim?
In Northern Ireland an extension of more than one storey can be permitted development at the rear if it projects no more than 3 metres and is at least 7 metres from the rear boundary. Two-storey side extensions, and any in a conservation area, need planning permission.
What drawings do I need?
Planning drawings for the application, then more detailed building regulations drawings and structural calculations for building control. We prepare both.
Do you build double-storey extensions in County Antrim?
Yes. We build double-storey extensions across County Antrim, including postcodes BT1, BT10, BT11, BT12 and BT13. Every project starts with a free home visit.