A rear extension in the Sevenoaks district is shaped by the plot as much as the house. On the 1930s and post-war semis of Swanley, Hextable and Dunton Green the usual project is a kitchen-diner pushed into a garden shared with a neighbour whose window is only a few metres away, so depth and daylight decide the design. The large detached and Edwardian houses of Sevenoaks town, Kippington and Wildernesse have more room but often an earlier extension already. Where the house is in the Green Belt, that earlier work counts against a floorspace allowance, so the new extension is sized from the original house, not from the garden.
The permitted development rules for rear extensions
On a house, a single-storey rear extension is permitted development if it projects no more than 3 m from the original rear wall on a terraced or semi-detached house, or 4 m on a detached house, is no higher than 4 m, has eaves no higher than 3 m where it comes within 2 m of a boundary, and uses materials similar in appearance to the house. The larger home extension route allows 6 m or 8 m through a 42-day prior approval, where the council consults the neighbours.
Flats and maisonettes have no householder permitted development rights, so any rear extension to a flat needs planning permission. Previous extensions count against the allowance, and the larger home extension route is not available in conservation areas.
What changes in Sevenoaks
In a conservation area a single-storey rear extension can still be permitted development within the 3 m or 4 m limit, but cladding it in materials that differ from the house is not, and the 6 m and 8 m prior approval route is closed.
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' 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. 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. 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. 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. 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. Source
Depth, height and daylight on a Sevenoaks semi
Sevenoaks' Residential Extensions guidance advises that, normally, a rear extension on a semi-detached or terraced house should not project more than 3 metres from the rear elevation, and on a detached house close to a neighbour generally no more than 4 metres. A single-storey extension within 2 metres of a boundary should normally be no more than 3 metres to the eaves and 4 metres overall. These are the same figures as the permitted development limits, but here they are the council's yardstick when a planning application is needed.
The guidance also says a side wall facing a neighbour should not normally contain windows, and that the council will not accept a window that overlooks the private amenity area next to the back of an adjoining house, which it normally takes as the first 5 metres from the back of that property. The 45-degree test is described as a useful guideline only: passing it does not make a proposal acceptable, and the council still looks at overshadowing and outlook. On a long, narrow semi garden we test the depth against all of these before fixing the plan.
How earlier additions count in the Green Belt
On a Green Belt house the council's Green Belt guidance measures a new extension against the "original" dwelling: the house as first built or, for older homes, as it stood on 1 July 1948. It says additions made since that date are considered cumulatively and count towards the overall increase, because small reductions in openness repeated many times can be harmful. Policy GB1 sets the 50% floorspace limit, measured externally, and the total includes previous extensions and alterations as well as outbuildings within 5 metres of the dwelling.
The guidance is explicit that 50% is a total over the life of the house and not an allowance per application, and that an addition can still be judged disproportionate below 50% if the design is unacceptable. A rear extension on a house that was extended by a previous owner is therefore measured from the original floorspace, and we establish that figure first.
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. 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.
Fees and timelines
Sevenoaks decides a householder application or a Lawful Development Certificate within 8 weeks of validation. The council fee is £528 for a householder application and £264 for a Lawful Development Certificate (England, from 1 April 2025; verify before applying).
Our own fee is fixed once quoted. The quote tool gives an indicative figure in minutes and a written quote follows within 24 hours.
Primary sources
- Sevenoaks District Council: Residential Extensions SPD (2009)
- Sevenoaks District Council: Development in the Green Belt SPD (2015)
- The Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2: the permitted development limits quoted above.
- Planning Portal: do you need permission: the national guidance and the application service every council accepts.
- Sevenoaks District Council: planning: the council's own constraints map, validation checklist and pre-application service.
