The permitted development rules for side extensions
A single-storey side extension is permitted development on a house if it is no wider than half the width of the original house, no higher than 4 m, with eaves no higher than 3 m where it comes within 2 m of a boundary, and finished in materials similar in appearance to the house.
Anything wider than half the original house, any two-storey side extension, and any side extension on a flat needs planning permission. Garage conversions that keep the walls are usually permitted development; extending beyond them is assessed as a side extension.
What changes in Sutton
Side extensions are not permitted development in a conservation area at all, so there a planning application is always required.
The local constraints that most often decide the route in Sutton are listed below, taken from our Sutton area page. Each one is a reason to confirm the address on Sutton's constraints map before relying on permitted development.
- Conservation areas. Sutton has numerous conservation areas, including Carshalton Village, Cheam Village, Sutton Garden Suburb and parts of Wallington and Beddington. Roof extensions and cladding in these areas need planning permission. Check the council's constraints map for your address before relying on permitted development. Source
- Article 4 directions. Some Sutton conservation areas, including Sutton Garden Suburb and parts of Carshalton, are understood to carry Article 4 directions removing permitted development for front alterations and boundary treatments. Coverage varies by area, so we confirm with the council before recommending a permitted development route.
- River Wandle flood zones. The Wandle rises at Carshalton Ponds and runs north through Carshalton, Hackbridge and Beddington. Land along it falls within Flood Zones 2 and 3, and basements or new ground-floor habitable rooms there may need a flood risk assessment. Source
- Green Belt and Metropolitan Open Land. The southern edge of the borough at Little Woodcote, Woodmansterne and Belmont includes Green Belt, and Beddington Park, Roundshaw Downs and other open spaces are Metropolitan Open Land. Extensions and outbuildings to houses adjoining these areas are judged on openness.
- Trees and tree preservation orders. Sutton's suburban streets and larger plots carry many protected trees. A tree survey and arboricultural method statement are often needed for rear extensions and garden rooms close to a protected tree or in a conservation area.
What Sutton planning officers look for
Sutton is a suburban borough and its officers are protective of the rhythm of 1930s streets. Side extensions that close the gap between semis, are flush with the front wall or carry a roof up to the main ridge are the most common refusal reason. Single-storey rear extensions of 3 to 4 m are generally acceptable; two-storey rear extensions are judged closely against the 45-degree rule and neighbours' outlook. Hip-to-gable lofts with rear dormers are routinely accepted outside conservation areas. Pre-application advice is available for a fee.
Fees and timelines
The council fee for a householder application is £528 and for a Lawful Development Certificate £264 (England, from 1 April 2025; verify before applying). Both are decided within 8 weeks of validation.
Our own fee for the drawings and the application is fixed once quoted and depends on the project type, its size and the constraints above. Answer six questions in the quote tool and we email you an indicative figure within minutes, then a written fixed-fee quote within 24 hours.
