What is a side extension?
A side extension builds into the space beside a semi-detached or detached house, often over a driveway or side passage. It can be single-storey, for a utility room, downstairs toilet or study, or two-storey to add a bedroom above.
How much does a side extension cost in Belfast?
In Belfast, a side extension typically costs £1,450–£2,300 per m² of floor area, including VAT at 20% (standard rate). For a typical 28 m² project that is roughly £40,500–£64,500. We start from a UK-wide guide and adjust it for Northern Ireland, where labour and materials are typically about 14% below average. Design, structural calculations and council fees are extra.
| Size or type | Guide range |
|---|---|
| Smaller project (20 m²) | £29,000–£46,000 |
| Typical project (28 m²) | £40,500–£64,500 |
| Larger project (39 m²) | £56,500–£89,500 |
| Rate per m² | £1,450–£2,300 |
The main things that change the cost:
- Single or two storeys
- Drains, gas meters or soil pipes that need moving
- Boundary walls and access for materials
- Internal fit-out of the new rooms
For a figure based on your own measurements, try the guide price tool, then book a free home visit for a fixed quote.
Do you need planning permission for a side extension in Belfast?
Approvals for a side extension in Belfast follow the rules for Northern Ireland. In Northern Ireland a single-storey side extension can be permitted development if it is no more than 4 metres high, no wider than half the original house and not beside a wall facing a road. Side extensions of more than one storey need planning permission.
In Northern Ireland building control approval comes from the council, usually through a full plans application or a building notice, with inspections during the work and a completion certificate at the end.
We check this for your address before design work starts, so there are no surprises later.
What we handle
What we take care of:
- Building control completion
- Opening into the existing house
- Survey of the boundary, drains and access
- Walls, roof and windows to match the house
- Planning drawings and application
- Foundations and drainage alterations
How long does it take?
Build time is typically 10 to 16 weeks on site depending on size and storeys. Design and approvals come first and typically take 3 to 4 months, depending on the approval route.
Building in Belfast
Across Northern Ireland, bungalows and chalet bungalows are common, so loft conversions and rear extensions are popular projects.
Projects in Belfast (BT1, BT2, BT3 and BT4) are surveyed in person before we quote, so the price reflects your actual property.
How it works
- 01
Free visit
We look at the property, talk through what you want and measure up. There is no charge and no obligation.
- 02
Drawings and written quote
We prepare the design and give you a written, itemised quote with inclusions and exclusions.
- 03
Permissions sorted
Applications, building control and neighbour arrangements, all before the start date.
- 04
Build and handover
Work follows the agreed programme, with inspections along the way and a completion certificate at the end.
Questions about side extensions in Belfast
How long does it take?
Roughly 10 to 16 weeks on site once approvals are in place, depending on whether it is one or two storeys.
Will I lose my driveway?
A side extension often takes the side drive. We look at whether parking can be kept at the front and whether a dropped kerb is needed.
Can a side extension go over a drain?
Often yes, with a build-over agreement from the water company or by rerouting the drain. We check this during the survey.
How much does a side extension cost in Belfast?
In Belfast, a side extension typically costs £1,450–£2,300 per m² of floor area, including VAT at 20% (standard rate). A typical 28 m² project comes to roughly £40,500–£64,500. Your fixed price depends on the survey and drawings.
Do I need planning permission for a side extension in Belfast?
In Northern Ireland a single-storey side extension can be permitted development if it is no more than 4 metres high, no wider than half the original house and not beside a wall facing a road. Side extensions of more than one storey need planning permission.
Do you build side extensions in Belfast?
Yes. We build side extensions across Belfast, including postcodes BT1, BT2, BT3 and BT4. Every project starts with a free home visit.