The 45 degree line and the first-floor cill
Brighton & Hove City Council sets out what it expects in its Design Guide for Extensions and Alterations, a supplementary planning document updated in January 2020. Read it before anyone draws a line.
Every rear extension is expected to pass a 45 degree test. Draw a line at that angle from the far corner of the new build towards your neighbour's nearest window. If the line crosses the middle of that window, the scheme may cost them daylight and more testing follows. Bathrooms and landings don't count.
Two smaller rules shape the roof. A pitched roof on a single-storey rear addition should sit below the cill of the first-floor windows, and a two-storey one should stay below the main ridge. The guide also wants the bulk kept low along the shared boundary, and on an infill it asks for a wall as close to 2 metres high as reasonably possible at the boundary line.
What you won't find is a maximum depth in metres, a garden percentage or a fixed gap for side extensions. Instead it asks for side additions to step back from the front, to drop below the ridge if they go up two floors, and to keep the spacing between houses so a street of semis doesn't close up into a terrace. So look at what was approved nearby on the Planning Register.
The guide welcomes contemporary design, even in conservation areas. Roof terraces on top are another matter. Anything that creates new overlooking is unlikely to get through.
Where the council has taken the rights away
Nationally, a house can usually go 3 metres out at the back on one storey (4 metres if detached) without applying, and twice that through the neighbour consultation scheme. Those rights belong to houses. A flat has none, so in a city where half of households live in one, plenty of people need an application for any addition at all.
More than 30 conservation areas cover large parts of the city, and part of it lies in the South Downs National Park, where the park authority handles planning. On that land, side extensions, two-storey rear extensions and cladding all need permission, and the larger 6 and 8 metre route is closed. A single-storey back addition within the basic limits is still possible on most houses there.
Not in The Avenues, though. The Article 4 direction there applies to all property, including parts of Cliftonville and The Drive, and says even small extensions and conservatories need consent. The Brunswick direction lists the same works. Patcham's, confirmed in 2012, covers the enlargement, improvement or alteration of a house. The Round Hill and West Hill directions are different. They deal with fronts facing the street, not the back garden.
Matching the job to the house
Roughly in the order I'd expect to see them.
Single-storey rear. The usual first step on a Preston or Round Hill terrace, and still permitted development on a house outside the Article 4 areas. Keep the roof below the upstairs window cills and check the 45 degree line against next door's back room.
Side return. Filling the passage beside a Victorian back addition counts as a side extension, so in a conservation area it needs permission. The council wants the wall on the boundary kept as near to 2 metres as it reasonably can be.
Kitchen. Glazing decides most of it. A flat roof with a rooflight often beats a lean-to that has to duck under the upstairs cill.
Open plan. Taking out the old back wall needs a steel beam, designed and signed off. Where the floors above are someone else's flat, agree the work with them first.
Wrap-around. Part side and part rear, so it always needs planning in a conservation area. The 45 degree test applies to the rear part, and the side part must not close the gap to next door.
Two-storey. Better suited to the suburban semis on the downland fringe at Patcham, Woodingdean and Hangleton, where there is room at the side. Expect officers to ask for a set-back from the front and a lower ridge.
Home office. The design guide asks for a fair share of garden to be kept after any rear addition. An attached study often uses a narrow plot better than a cabin at the end.
Ground-floor bedroom. Round Hill and other hillside streets make level access hard. Plan the floor height and any ramp before the planning drawings go in.
Planning. Three questions first. House or flat? Conservation area or national park? One of the Article 4 areas?
Cost. No figure from us. Deep footings on chalk, a steel in a flat conversion and conservation materials all move the price. Three quotes will show you how far.
Chalk, groundwater and elms
Most of the city stands on chalk, with clay with flints on the higher ground. Chalk dissolves. When the A27 bypass was cut along the northern edge, engineers found many buried hollows in it, and pipes of softer material under the clay with flints gave trouble. The British Geological Survey's karst study warns that such features can cause uneven settlement. Soft pockets in a trench mean calling the engineer back.
There are no main rivers in the city. The sea is the risk at Portslade-by-Sea and the Marina, and surface water everywhere else. Groundwater rises closest to the surface in Aldrington, Patcham, Falmer and along parts of the coast, and the council's flood study recommends a flood risk assessment for any development where groundwater risk is identified, whatever the size.
Trees count too. In a conservation area, any tree with a stem over 75mm across at 1.5 metres up needs six weeks' written notice before work. The city also has more than 17,000 elms, planted because they cope with thin chalk soil. Check the council's tree map before setting out.
Where these facts come from
- Design Guide for Extensions and Alterations SPD (Brighton & Hove, 2020)
- Permitted development rights for householders, technical guidance (GOV.UK)
- Brighton & Hove City Council: Article 4 directions
- Article 4 direction: The Avenues
- Article 4 direction: Patcham
- Brighton & Hove Level 1 and 2 Strategic Flood Risk Assessment (2018)
- Caves and karst of the Chalk in East Sussex (BGS)
- Brighton & Hove City Council: Tree Preservation Orders
- Brighton & Hove City Council: National Elm Collection
- Brighton & Hove City Council: Census 2021 city profile
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Questions people in Brighton ask
I'm in The Avenues in Hove. Can I add a small conservatory without applying?
No. The Article 4 direction there names conservatories, porches, sheds and small extensions as needing consent.
Our ground-floor flat has a garden. Can we use the 3 metre rule?
No. Householder permitted development applies to houses, not flats, so a rear extension to a flat needs a planning application.
We're in Patcham. Does the groundwater issue affect a normal kitchen extension?
Patcham is one of the places the council's flood study flags for high groundwater. For a single-storey extension on standard footings it is mainly a design point. If you plan a basement or a sunken floor, ask about a flood risk assessment early.
Next door's window is close to our boundary. How do we know if we pass the 45 degree test?
Ask your designer to draw it on the plan. The line runs from the outer corner of the extension towards the neighbour's window. If it passes the window's centre, the council may want more daylight testing or a smaller scheme.