What is a basement conversion?
A basement conversion turns an existing cellar into living space, or excavates a new basement under the house or garden. Waterproofing, structural support and daylight are the main design challenges.
How much does a basement conversion cost in Antrim?
In Antrim, a basement conversion typically costs £1,650–£2,750 per m² for converting an existing cellar, including VAT at 20% (standard rate). For a typical 30 m² project that is roughly £49,500–£82,500. The guide for Antrim reflects typical costs in Northern Ireland, roughly 14% below the UK-wide figure. Design, structural calculations and council fees are extra.
| Size or type | Guide range |
|---|---|
| Smaller project (21 m²) | £34,500–£58,000 |
| Typical project (30 m²) | £49,500–£82,500 |
| Larger project (42 m²) | £69,500–£115,500 |
| Rate per m² | £1,650–£2,750 |
What moves the price most:
- Ground water and access
- Waterproofing system
- Lightwells
- Existing cellar or new excavation
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 basement conversion in Antrim?
The approval route for Antrim is set by planning law in Northern Ireland. In Northern Ireland converting an existing cellar usually does not need planning permission; new excavation and external changes may.
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.
At the survey we check your house’s planning history and any restrictions, and tell you which route applies.
What we handle
From survey to sign-off, the job normally involves:
- Written agreement with neighbours on shared walls
- Lightwells and windows
- Underpinning or new structure where needed
- Waterproofing design (tanking or cavity drain membrane)
- Insulation, ventilation and drainage pumps
- Electrics, heating and plastering
How long does it take?
Build time is typically 8 to 16 weeks for a cellar conversion; longer for new excavation. Before that, surveys, drawings and approvals usually take 3 to 5 months, longer if the council asks for changes.
Building in Antrim
Northern Ireland has many detached and semi-detached homes on generous plots, which suits single-storey rear extensions and garage conversions.
We work across Antrim and neighbouring areas such as Ballymena, Newtownabbey and Belfast. Postcodes we cover in Antrim include BT41.
How it works
- 01
Visit and measured survey
A free visit to understand the project, check access and take measurements.
- 02
Design and fixed quote
Drawings, structural calculations and an itemised, fixed price for the agreed specification.
- 03
Approvals in place
We handle planning, building control and neighbour notices before work starts.
- 04
Build and handover
Work follows the agreed programme, with inspections along the way and a completion certificate at the end.
Questions about basement conversions in Antrim
Is new excavation worth it?
It can be in high-value areas, but costs are high. We give a feasibility view before design starts.
Do I need planning permission for a basement conversion in Antrim?
In Northern Ireland converting an existing cellar usually does not need planning permission; new excavation and external changes may.
Who approves a basement conversion in Antrim?
Planning matters in Antrim go to the council covering your address, and the work needs building control approval. We prepare and submit the applications for you.
How much does a basement conversion cost in Antrim?
In Antrim, a basement conversion typically costs £1,650–£2,750 per m² for converting an existing cellar, including VAT at 20% (standard rate). A typical 30 m² project comes to roughly £49,500–£82,500. Your fixed price depends on the survey and drawings.
Can a basement be a bedroom?
Yes, with a suitable escape route such as a lightwell window or protected stair.
How long does it take?
A cellar conversion takes around 8 to 16 weeks; new basements take longer.