Four metres out the back, on any type of house
Advice written for England quotes the wrong numbers here. Cardiff works to the Welsh order. Here a single storey rear addition can go up to 4 metres beyond the original back wall and 4 metres high without a planning application, and that applies to a mid terrace in Splott just as it does to a bungalow in Cyncoed. There is no separate shorter limit for terraces and semis.
Go up a floor and the Welsh rules get tighter. The ground floor can still project 4 metres but the upper floor only 3, and the whole thing must sit at least 10.5 metres from the back boundary. Plenty of Cardiff gardens are shorter than that. Within 2 metres of any boundary the eaves can't top 3 metres. Nothing may cover more than half the land around the original house, which means the house as built or as it stood on 1 July 1948.
Conservation areas change the picture. Across Cardiff's 27, from Cathedral Road and Roath Park to Llandaff, Whitchurch's Church Road and Rhiwbina Garden Village, nothing over one storey is permitted development. A single storey side addition is capped at 3 metres wide and must sit a metre back from the front wall, and cladding the house in render, timber or stone needs consent. Rhiwbina has had its own Article 4 direction since 1986 covering roof alterations, so tying a new roof into the old slopes there is a planning matter from the start.
Annexes, side returns and the council's 2017 guide
Cardiff's Residential Extensions and Alterations guidance from November 2017 is the document officers quote when an application is needed. It notes that many terraces in the city were built with a two or three storey back annexe, which leaves a narrow strip down the side. Filling that strip is the classic Roath and Splott project.
The guide gives no fixed depth. It uses light tests instead. Draw a line at 45 degrees from the middle of the neighbour's nearest window, on plan and in elevation, and the new wall shouldn't cross it. A second test looks up at 25 degrees from the same point. Two storey additions at the back should normally stay 2 metres from a party wall boundary and a metre from others. Keep at least 25 square metres of usable garden, with room for bins and bikes.
Before anyone digs, find the drains. Welsh Water's consent is needed to build over, or within 3 metres of, a public sewer or lateral drain, and on a terrace the shared run often crosses the back yards.
Choosing the right build for the house you have
Side return. Made for the annexed terraces of the inner suburbs. The council prefers a pitched lean-to roof kept low along the boundary, and glass rather than brick on the side facing next door.
Single storey rear. The 4 metre Welsh allowance is the same for every house type. Check the planning history first, because a condition on the original consent can take the right away.
Wrap-around. For an annexed terrace this means filling the side strip and going past the end of the annexe in one go. It almost always needs an application, and officers will run the light tests against both neighbours.
Kitchen. Most side returns in the city end up as kitchens. Plan rooflights early if the build darkens the old back room.
Open plan. Knocking the old back room, the annexe and the new space together means steel across load-bearing walls. The Party Wall Act applies here as in England, so serve notice before the beams go in.
Two storey. Best suited to the semis and detached houses further north. For a semi the guide asks for the upper floor to be set back from the front and set down from the ridge so the pair still reads as a pair.
Ground floor bedroom. The 2017 guide itself points to level access and a downstairs bathroom as features that suit changing needs. A room for a relative is treated as an annexe, so it must stay modest and share the kitchen, garden or entrance with the house.
Home office. Large garden buildings used for more than storage get a cool reception from Cardiff officers. An attached study avoids the argument, and it keeps the garden above the 25 square metre guide.
Planning. If you think a scheme falls within the Welsh limits, the Welsh Government suggests a Lawful Development Certificate.
Cost. No Cardiff figures here. A flood assessment, a sewer agreement or deeper foundations on soft ground can each change the budget, so ask every builder what they've allowed for.
Soft clay by the bay, and the flood map
The British Geological Survey describes much of southern Cardiff as old tidal flats. That's soft blue clay with peat at the base, thickest under the former East Moors steelworks. Further north the city sits on stiff glacial till. Ask your engineer what the ground needs before you settle on a foundation design.
Natural Resources Wales publishes a Flood Map for Planning, updated each May and November. Around 10,000 Cardiff properties sit behind tidal defences. If your house falls in Zone 2, Zone 3 or a defended zone, any application needs a Flood Consequences Assessment scaled to the job, under the Welsh flood guidance in force since 31 March 2025.
Where these facts come from
- Welsh Government: planning permission for extensions
- Welsh Government: permitted development for householders, technical guidance
- Cardiff Council: Residential Extensions and Alterations SPG (2017)
- London Gazette: Rhiwbina Garden Village Article 4 direction (1986)
- Dลตr Cymru Welsh Water: building over or near sewers
- Natural Resources Wales: Flood Map for Planning
- Welsh Government: TAN 15 frequently asked questions
- British Geological Survey: Cardiff superficial geology
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Questions people in Cardiff ask
We live in a Splott terrace. Can we really build 4 metres out without applying?
Usually, yes. Welsh permitted development allows a single storey rear addition of up to 4 metres on any house, terraced included, as long as it stays under 4 metres high, keeps eaves to 3 metres near a boundary and the house isn't covered by a condition or Article 4 direction that removes the right.
Our neighbour in Roath already filled their side return. Does that help us?
It often does. Cardiff's guidance says the neighbour's existing infill is one of the things officers weigh.
There's a manhole in our back yard. Is that a problem?
Possibly. Building over or within 3 metres of a public sewer needs Welsh Water's agreement, and they rarely allow it over pipes wider than 225mm or deeper than 3 metres. Find out early.
We're in Rhiwbina Garden Village. What's different for us?
It is a conservation area, so nothing over one storey counts as permitted development, side additions are limited and new cladding needs consent. A 1986 Article 4 direction also covers roof alterations. Expect to apply, and match the existing finishes.