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 Sevenoaks
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 Sevenoaks are listed below, taken from our Sevenoaks area page. Each one is a reason to confirm the address on Sevenoaks's constraints map before relying on permitted development.
- Green Belt. Most of the Sevenoaks district outside the built-up areas of Sevenoaks town, Swanley, Westerham, Edenbridge and the larger villages is Metropolitan Green Belt. Extensions to houses in the Green Belt must not be disproportionate to the original dwelling, and outbuildings face a stricter test on size and siting. Check the council's constraints map for whether your plot is inside or outside the boundary. Source
- Kent Downs National Landscape. The Kent Downs National Landscape (formerly the Kent Downs AONB) covers the North Downs across much of the district, including Otford, Kemsing, Shoreham, Westerham and the countryside north of Sevenoaks town. It counts as Article 2(3) land, so permitted development is tighter: side extensions, two-storey rear extensions, roof extensions and the larger home extension route are excluded, outbuildings more than 20 m from the house are limited to 10 m², and external cladding is not PD. Officers also weigh landscape impact and materials more heavily. Source
- Conservation areas. Sevenoaks has around 40 conservation areas, including Sevenoaks High Street and The Vine, Westerham, Otford, Kemsing, Shoreham, Edenbridge, Riverhead, Chevening and many smaller village centres. Roof extensions are not permitted development inside them, and officers apply the character appraisal to extensions and dormers. Check the council's constraints map for the exact boundaries before relying on permitted development. Source
- Knole Park and listed buildings. Knole House and its deer park sit on the edge of Sevenoaks town, and the town, Westerham, Otford, Shoreham, Edenbridge and the villages contain a very high concentration of listed houses, cottages and churches. Listed building consent is needed for almost any alteration, internal or external, and houses within the setting of Knole or a listed neighbour face closer scrutiny on design. Source
- River Darent and River Eden flood risk. The River Darent runs from Westerham through Riverhead, Otford and Shoreham, and the River Eden runs through Edenbridge; land alongside both sits in Flood Zones 2 and 3. Ground-floor extensions and basements there may need a flood risk statement with the application. Check the government flood map for your address. Source
What Sevenoaks planning officers look for
Sevenoaks officers assess householder applications against the Core Strategy 2011, the Allocations and Development Management Plan 2015 and the council's residential extensions guidance, with Green Belt and Kent Downs policy in the background for most of the district. They tend to look for extensions that stay subordinate to the original house in scale and roof form, side extensions set back from the front wall and below the ridge with a gap kept to the boundary, rear extensions that clear the 45-degree line to the neighbour's nearest habitable window, and dormers set back from the eaves and clad in matching tile. In the Green Belt the first question is whether the extension, added to anything built since the house was original, is disproportionate; officers usually measure this by floorspace and volume, and the council's guidance has historically treated additions beyond about half the original as needing a strong case. In the Kent Downs they look for materials and roof forms that sit quietly in the landscape. Common refusal reasons are disproportionate Green Belt additions, side extensions that create a terracing effect, and roof extensions or box dormers in conservation areas. Outside the designations, hip-to-gable and rear-dormer lofts within 50 m³ on a semi are routinely lawful and we secure a Lawful Development Certificate as proof. Sevenoaks offers a paid pre-application service. Decisions can run past the statutory 8 weeks; allow 8–12 weeks.
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.
