Welsh rules, not English ones
Plenty of advice online about lofts is written for England. Cardiff follows the Welsh version of permitted development, set out in a 2013 Welsh Government order, and it differs in places that catch people out. A rear dormer can still go ahead without a planning application if it adds no more than 40 cubic metres to a terrace, or 50 to any other house. It must stay below the ridge, keep off the front roof slope and sit at least 20cm back from the eaves. Any window facing sideways within 10.5 metres of a boundary has to be obscure glazed and fixed below 1.7 metres.
Materials have to match the house as far as practicable. Welsh Government guidance says plainly that uPVC cladding on a dormer won't be accepted. On most Cardiff terraces that points to slate, and the council's approved schemes often use vertically hung slate on the dormer cheeks.
Then there are conservation areas. In Wales a house in one gets no permitted development for any roof enlargement, and none for rooflights either. Even a single Velux in the back slope needs planning permission. Cardiff has 27 conservation areas, so this affects more households than you might think.
Terraces built for the coal boom
Cardiff had 1,870 people in 1801 and 164,333 by 1901. Coal through the Bute docks paid for it, and the houses from that boom are still the inner suburbs. Roath was a village of about 60 houses in 1841, had 524 by 1861, and the Tredegar and Bute estates kept building there for decades. Splott's tight rows went up from the 1880s for workers at the East Moors steelworks. Roughly three in ten Cardiff households live in a terrace, above the Welsh average.
The council's own extensions guide notes that many Cardiff terraces have an original two or three storey rear annexe. That shapes the loft. The main roof gives you one room and the annexe roof is where people look for a second. Officers are wary of dormers alongside annexes where permission is needed, because of overlooking into the next garden.
Up the hill it's a different stock. Whitchurch and Cyncoed have whole streets of bungalows, and turning one into a chalet with a big rear dormer is a regular item at planning committee. One recent Whitchurch case drew a petition from dozens of neighbours. It was approved, but only with obscure glass, because the back windows fell just short of the council's 10.5 metre guide to the gardens behind. Measure that distance before anyone draws anything.
Which conversion suits which Cardiff house
Rear dormer. The normal choice for a Roath or Splott terrace. Cardiff's planning guidance wants it set down from the ridge and in from the sides, with the windows filling most of the face. Big flat-roofed boxes that make a house look top-heavy are what the officers push back on.
Rooflights only. Outside a conservation area this is the least fuss. Inside one, Welsh rules mean an application even for rooflights, and the council will want a few flush conservation-style units placed where they can't be seen from the street.
Hip to gable. Common on the semis and bungalows of north Cardiff. It's exempt from the 20cm eaves rule, and on Kelston Road in Whitchurch the officer said a hip to gable appeared to meet the Welsh criteria on its own. Add a rear dormer and the volumes count together.
Read our full guide: Hip-to-Gable Loft Conversions: What It Involves and Which Houses It Suits โ
L-shaped dormer. The obvious idea on a terrace with a back annexe. Get the main dormer and the annexe section measured as one volume against the 40 cubic metre limit, and expect the side windows to be the sticking point.
Planning. Check three things before paying for drawings. Is the house in a conservation area, has an earlier consent removed permitted development, and does the volume fit? If any answer is unclear, ask the council before you commit.
Extra bedroom. The 2021 Census found Cardiff had the highest share of households in Wales short of the bedrooms they need. Building control looks at the stairs and the fire escape route before signing off.
En-suite. A side window that must be obscure glazed is no hardship in a bathroom, which is why the shower room often goes on the side nearest the neighbours. Work out the soil pipe route first.
Mansard. Always a full application, and a hard sell in Cardiff. The council's guidance favours dormers that look subservient to the roof. In Llandaff the appraisal says even large box dormers are unlikely to be supported.
Home office. Worth thinking about where a conservation area rules out a dormer. A room with sloping ceilings and rooflights can be too low for a bedroom yet fine for a desk.
Read our full guide: Loft Home-Office Conversion: What Works Well Up There and What to Specify โ
Cost. We don't publish a Cardiff figure. A terrace in Splott and a stone villa on Cathedral Road are different jobs. Three site visits will tell you more than any table.
Slate, stone and sign-off
The conservation areas cover streets like Cathedral Road, Roath Park, Wordsworth Avenue, Oakfield Street, Conway Road and Llandaff, plus old villages such as St Fagans and St Mellons. On Cathedral Road the late Victorian houses are Pennant sandstone with Bath stone dressings under Welsh slate. Llandaff's 2020 appraisal wants natural slate kept or replaced with reclaimed or new slate, and an Article 4 direction there also covers roof materials facing the road.
Search your street on the council's planning system, which has an A to Z street search. Building control is Cardiff Council at County Hall (buildingcontrol@cardiff.gov.uk) unless your builder uses a registered building control approver. The Party Wall Act applies in Wales, so on a terrace or semi your neighbours need notice before steels go into the shared wall. Keep the completion certificate with your deeds.
Where these facts come from
- Welsh Government: planning permission for loft conversions
- Welsh Government: permitted development for householders, technical guidance
- Welsh Government: building regulations for loft conversions
- Cardiff Council: Residential Extensions and Alterations SPG (2017)
- Cardiff Council: Llandaff Conservation Area Appraisal (2020)
- Welsh Government: Housing in Wales, Census 2021
- Building (Amendment) (Wales) Regulations 2013: fire suppression
- Roath Local History Society: the streets of Roath
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Questions people in Cardiff ask
I live in the Roath Park conservation area. Can I just put Velux windows in the back?
Not without planning permission. Welsh permitted development doesn't cover rooflights on a house in a conservation area, front or back. The council usually accepts a few flush conservation rooflights placed out of sight of the street.
Can I turn my loft into a separate flat to let?
Cardiff has more private renting than anywhere else in Wales, so it's a fair question. A self-contained flat is a new dwelling, though, which needs full planning permission. Since January 2016 Welsh Building Regulations have also required sprinklers in new flats, including ones made by converting part of a house. Budget for both from day one.
My bungalow in Whitchurch is surrounded by others. Will the neighbours stop a dormer?
They can object, and in one Whitchurch case dozens signed a petition. What decides it is the planning guidance. Keep rear windows at least 10.5 metres from the gardens behind, or accept obscure glass, and keep the dormer below the ridge and in from the sides.
We bought a new house at Plasdลตr. Can we convert the loft?
Possibly. Estate permissions sometimes remove permitted development, so read your house's conditions on the council's planning system first. If the roof is built from factory trusses, the conversion will need a steel frame and a structural engineer.