What is a mansard loft conversion?
A mansard loft conversion rebuilds the roof with a nearly vertical slope (about 72 degrees) and a flat top, effectively adding a new storey. Small dormer windows sit within the steep slope. Mansards are common on period terraces in towns and cities.
How much does a mansard loft conversion cost in County Londonderry?
In County Londonderry, a mansard loft conversion typically costs £56,000–£81,500 for a typical rear mansard, including VAT at 20% (standard rate). Building costs in Northern Ireland run about 14% below the UK average, and the figures for County Londonderry are adjusted to reflect that. Design, structural calculations and council fees are extra.
| Size or type | Guide range |
|---|---|
| A typical rear mansard | £56,000–£81,500 |
| Higher specification | £81,500–£102,000 |
The main things that change the cost:
- Agreements with neighbours on shared walls
- Roof covering: natural slate costs more
- Bathrooms and finishes
- Number of dormers and lead work
Use the guide price tool to see how size and specification change the figure, then ask for a written, itemised quote.
Do you need planning permission for a mansard loft conversion in County Londonderry?
Here is how the rules work for a mansard loft conversion in Northern Ireland. In Northern Ireland a mansard needs planning permission from the council.
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
From survey to sign-off, the job normally involves:
- Building control sign-off
- Planning drawings and application
- Staircase and fire-safe escape route
- Insulation, electrics, plumbing and plastering
- New roof structure, slate or tile covering and lead work
- Structural design and calculations
How long does it take?
The build itself usually takes 9 to 12 weeks on site, after planning approval. Allow time beforehand for drawings and approvals, usually 3 to 4 months.
Building in County Londonderry
Northern Ireland has many detached and semi-detached homes on generous plots, which suits single-storey rear extensions and garage conversions.
Towns we cover in County Londonderry include Derry, Coleraine, Limavady and Magherafelt.
How it works
- 01
Visit and measured survey
A free visit to understand the project, check access and take measurements.
- 02
Design and pricing
Design options, then a fixed quote based on the chosen drawings and specification.
- 03
Permissions sorted
Applications, building control and neighbour arrangements, all before the start date.
- 04
Construction and completion
We build, put right any snags and hand over the building control certificate.
Questions about mansard loft conversions in County Londonderry
Do I need planning permission for a mansard loft conversion in County Londonderry?
In Northern Ireland a mansard needs planning permission from the council.
Do you build mansard loft conversions in County Londonderry?
Yes. We build mansard loft conversions across County Londonderry, including postcodes BT28, BT45, BT46, BT47 and BT48. Every project starts with a free home visit.
Why choose a mansard over a dormer?
A mansard gives more usable floor space and a neater appearance from the street, but costs more and needs planning permission.
What roof covering is used?
Natural slate is traditional; some councils accept other materials. We follow local guidance.
How much does a mansard loft conversion cost in County Londonderry?
In County Londonderry, a mansard loft conversion typically costs £56,000–£81,500 for a typical rear mansard, including VAT at 20% (standard rate). Your fixed price depends on the survey and drawings.
Can I stay at home?
Usually. The new roof is built under a temporary cover so the house stays dry.