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 Cardiff?
In Cardiff, a mansard loft conversion typically costs £60,000–£87,500 for a typical rear mansard, including VAT at 20% (standard rate). We start from a UK-wide guide and adjust it for Wales, where labour and materials are typically about 8% below average. Design, structural calculations and council fees are extra.
| Size or type | Guide range |
|---|---|
| A typical rear mansard | £60,000–£87,500 |
| Higher specification | £87,500–£109,500 |
What moves the price most:
- Roof covering: natural slate costs more
- Party wall surveyor fees
- Front, rear or both
- Number of dormers and lead work
A guide range is only a starting point: the guide price tool gives an instant figure for your size, and a free survey turns it into a fixed quote.
Do you need planning permission for a mansard loft conversion in Cardiff?
Whether you need planning permission depends on your house and its location in Wales. In Wales a mansard roof needs planning permission and is judged against local design guidance.
Building regulations in Wales are set by the Welsh Government. Approval comes from the council’s building control service or a registered building control approver, with inspections during the work and a completion certificate at the end.
If the answer is not clear-cut, we can apply for formal confirmation before you commit to the build.
What we handle
From survey to sign-off, the job normally involves:
- Building control sign-off
- Dormer windows
- New roof structure, slate or tile covering and lead work
- Staircase and fire-safe escape route
- Planning drawings and application
- Insulation, electrics, plumbing and plastering
How long does it take?
Expect 9 to 12 weeks on site, after planning approval. Allow time beforehand for drawings and approvals, usually 3 to 4 months.
Building in Cardiff
Terraced houses in Welsh towns often have narrow plots, which suits rear and side return extensions and rear dormers.
Cardiff sits in South Glamorgan. We cover the whole of Cardiff, including postcodes CF10, CF11, CF14 and CF15.
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
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 mansard loft conversions in Cardiff
How long does a mansard loft conversion take in Cardiff?
Typically 9 to 12 weeks on site, after planning approval. Design and approvals beforehand usually take 3 to 4 months.
What roof covering is used?
Natural slate is traditional; some councils accept other materials. We follow local guidance.
Do I need party wall agreements?
Yes, mansards on terraced houses almost always involve both party walls.
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.
Do I need planning permission for a mansard loft conversion in Cardiff?
In Wales a mansard roof needs planning permission and is judged against local design guidance.
How much does a mansard loft conversion cost in Cardiff?
In Cardiff, a mansard loft conversion typically costs £60,000–£87,500 for a typical rear mansard, including VAT at 20% (standard rate). Your fixed price depends on the survey and drawings.