Bristol's own rule book: 3.5 metres and a 45 degree line
Once a scheme goes beyond permitted development, Bristol City Council judges it against SPD2, its guide to designing house alterations and extensions. It dates from 2005 and is still on the council's list of planning guidance.
For a single-storey rear addition the guide sets a maximum depth of 3.5 metres, and says terraced houses may need much less. Two storeys at the back are "unlikely to be acceptable beyond 2.75 metres", and the guide expects them to be ruled out on many narrow terraces. Then comes the angle test. Draw a 45 degree line, on plan and in elevation, from the centre of the nearest ground-floor window of a habitable room next door. Your extension shouldn't cross it, though an existing tall boundary wall can earn some leeway.
Side additions have their own figures. On a semi, the guide wants the new part set back at least 1 metre from the front wall with a lower ridge, so the original pair still reads as a pair. Keep at least 1 metre to the side boundary, more where the street already has wider gaps. The council's worry is a terracing effect, where a row of semis slowly joins up. End-of-terrace houses are the exception. There the guide asks you to carry on the terrace's building line and ridge height.
Facing windows should generally be 21 metres apart, less in the tightly packed inner areas. Get the 45 degree line drawn on the first sketch, not after the steel is designed.
Coal, clay and old river channels
What sits under the patio varies across the city. The British Geological Survey records shrink-swell clays in the Mercia Mudstone under Redland and in the Lower Jurassic rocks of Horfield and south Bristol, so trees near a new footing need thinking about. In Redcliffe, Bedminster and Southville, weathered Redcliffe Sandstone can cause local subsidence.
East and south Bristol were coalfield. In a development high risk area GOV.UK says you must check for mining risks, and you need a permit from the Mining Remediation Authority before foundations or new drains cut into coal or a mine entry. Building control can ask for a coal mining risk assessment even on a kitchen extension.
Flooding is concentrated. The council's 2023 flood study picks out Avonmouth and St Philip's Marsh, Ashton Vale, Eastville along the Frome, and the Avon banks at Sea Mills, Shirehampton and Totterdown. If your house is in Flood Zone 2 or 3, the application needs a flood risk assessment, and the Environment Agency's standing advice asks for a raised floor or flood-resilient construction.
Choosing a type for a Bristol house
Ordered by how often each one works without a fight.
Single-storey rear. Outside conservation areas, 3 metres on a terrace or semi and 4 on a detached house is permitted development. On a full application the council's guide stops at 3.5.
Kitchen. Plenty of Bristol gardens rise or fall away from the back door. The step from the old floor to the new one, and where the glazing faces, decide whether the room works.
Side return. Filling the passage beside a Victorian back wing counts as a side extension. In Bedminster or any other of the 33 conservation areas, that means an application.
Open plan. Much of east Bristol was built in Pennant sandstone quarried at Fishponds, Stapleton and Hanham. Taking out a stone back wall needs an engineer's beam and proper padstones, not a guess.
Wrap-around. Part side, part rear, so the 1 metre gap and the 45 degree line both apply. It suits the wider plots of interwar semis in Knowle or Southmead better than a narrow terrace.
Two-storey. Expect an application and a depth near the 2.75 metre guide figure at the back. To the side of a semi, plan on the 1 metre set-back and a lower ridge from the start.
Home office. A low rear addition off the kitchen shares the heating and usually passes the angle test without trouble.
Ground-floor bedroom. On a sloping plot, level access is the hard part. In a flood zone the raised floor the Environment Agency wants has to be squared with a step-free entrance.
Planning. First check for a conservation area or Article 4 direction. Flats have no permitted development at all. A lawful development certificate puts the answer in writing.
Cost. No Bristol figure here. Sloping gardens and mining checks move the price too much for an average to help. Three quotes will show you the spread.
Where Bristol says no to permitted development
In a conservation area the national rules take away side extensions and anything over one storey at the back. Cladding goes too, and so does the larger 6 or 8 metre route. Two old Article 4 directions go further. At Henbury (1971) and on land at Sherrin Way (1982), alterations and extensions to a house need permission. The council's Pinpoint map shows the boundaries.
Sea Mills deserves a note of its own. The 1920s garden suburb is a conservation area, and its appraisal lists extensions that break the uniform size of the houses, or their planned front and rear building lines, as a threat. It also values the rhythm of gaps between the houses. A side addition there faces a hard test.
Trees in conservation areas over 75mm across need six weeks' notice before work, unless the work is needed for a full planning permission.
Where these facts come from
- Bristol SPD2: a guide for designing house alterations and extensions (2005)
- Bristol City Council: supplementary planning documents and guidance
- Bristol City Council: Article 4 directions
- Sea Mills conservation area character appraisal
- Bristol City Council: trees in conservation areas
- Bristol Strategic Flood Risk Assessment Level 2 (2023)
- Flood risk assessment: standing advice (GOV.UK)
- Home extensions in former coal mining areas (GOV.UK)
- BGS: geology of the Bristol district
- Permitted development rights for householders, technical guidance (GOV.UK)
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Questions people in Bristol ask
The council's guide says 3.5 metres, but I've heard I can go 6 metres. Which is right?
Both, for different routes. The 6 metre figure (8 for detached) is the national neighbour consultation scheme, and it doesn't apply in a conservation area. SPD2's 3.5 metres is what the council works to on a planning application.
We're in Totterdown near the river. Do we need a flood risk assessment?
Only if the house is in Flood Zone 2 or 3, so check the Environment Agency map first. The council's flood study does name the Avon at Totterdown. If you are in, the assessment goes in with the application.
Our semi is in Henbury. Can we build a rear extension without applying?
Probably not. A 1971 Article 4 direction there requires planning permission for alterations and extensions to houses. Check the council's Pinpoint map to see whether your street is inside it.
Is coal mining an issue for an extension in Fishponds or Bedminster?
It can be. Both sides of the city sit on the old coalfield. Check the Mining Remediation Authority map, and if the site is in a high risk area, get its permit before digging foundations that could meet old workings.