Typical prices by conversion type
The biggest factor in your budget is the style of conversion your roof allows. Cardiff has a huge stock of Victorian and Edwardian terraces in areas like Canton, Roath and Grangetown, and these often have generous roof heights that suit a dormer conversion. Post-war semis in suburbs such as Whitchurch, Rhiwbina and Llanishen frequently have hipped roofs, which usually means a hip to gable conversion to create usable space.
As a rough guide for the Cardiff area, a simple rooflight or Velux conversion tends to start around £25,000 to £35,000. A rear dormer, the most popular choice for terraces, typically runs £40,000 to £55,000. A hip to gable conversion is usually £50,000 to £65,000, and a full mansard, which reshapes most of the roof, can reach £60,000 to £80,000 or more. These figures assume a habitable room with insulation, a proper staircase, electrics and plastering, not just boarded storage space.
- Rooflight or Velux conversion: roughly £25,000 to £35,000
- Rear dormer conversion: roughly £40,000 to £55,000
- Hip to gable conversion: roughly £50,000 to £65,000
- Mansard conversion: roughly £60,000 to £80,000 plus
What pushes the price up or down
Two identical-looking houses can produce very different quotes. Head height is the first thing to check: you want around 2.2 metres from the top of the existing ceiling joists to the ridge. Many older Cardiff terraces pass comfortably, but if yours falls short, lowering ceilings below adds significant cost. The condition of the existing roof matters too. If the timbers or slates need substantial work, or there is historic damp, that gets addressed before anything else.
Adding an en suite typically adds £4,000 to £8,000 depending on the specification and how far pipework has to run. Structural steel, which most dormers need, varies with span and access; tight terraced streets with no rear lane access can add labour time simply because materials have to come through the house. Finally, your choice of finishes, from windows and flooring to fitted storage under the eaves, can swing the total by several thousand pounds either way.
Planning permission and building regulations in Cardiff
Many loft conversions fall under permitted development, meaning no full planning application is needed, but there are limits on volume and design, and dormers to the front of the house usually do need permission. If you live in one of Cardiff's conservation areas, such as parts of Pontcanna or Penarth's neighbouring streets if you are just outside the city, the rules are tighter, so check with the council early.
Building regulations approval is not optional. Cardiff Council's building control, or an approved inspector, must sign off the structural work, fire safety measures, insulation and staircase. Budget around £1,500 to £3,000 for structural engineer calculations, drawings and inspection fees on top of the build cost. If your house is a terrace or semi, you will likely also need a party wall agreement with your neighbours, which is usually straightforward but worth arranging before work starts.
How to budget sensibly and compare quotes
Get at least three itemised quotes and make sure they cover the same scope: structural steel, staircase, insulation to current standards, electrics, plumbing if applicable, plastering and making good. A quote that looks cheap often excludes decoration, flooring or scaffolding, so ask what is not included. A written schedule of payments tied to completed stages is a reasonable ask, and no reputable builder should want a large sum up front.
It is sensible to hold back a contingency of around 10 to 15 percent for the unknowns that only appear once the roof is opened up, such as tired timbers or old wiring that needs bringing up to standard. A well-built loft conversion remains one of the strongest-value improvements for Cardiff homes, typically adding a double bedroom and often a bathroom without losing any garden.
Published 31 July 2026 · BBS Wales