The permitted development rules for two-storey extensions
A two-storey rear extension is permitted development only within tight limits: no more than 3 m deep, at least 7 m from the rear boundary, no higher than the existing house, eaves matching the existing eaves, materials similar in appearance, and any upper-floor side window obscure-glazed and non-opening below 1.7 m.
Most two-storey extensions exceed one of those limits, most often the 7 m rear boundary rule on London plots, so the usual route is a householder planning application. Flats have no permitted development rights.
What changes in Dartford
Extensions of more than one storey are not permitted development in a conservation area, so planning permission is always needed there.
The local constraints that most often decide the route in Dartford are listed below, taken from our Dartford area page. Each one is a reason to confirm the address on Dartford's constraints map before relying on permitted development.
- Thames flood zone north of the A206. Land north of the A206 from Joyce Green and the northern edge of Temple Hill through Stone, Greenhithe and the Swanscombe marshes sits in Flood Zone 3 behind the Thames tidal defences. The area is defended, but ground-floor extensions, basements and any new habitable room at ground level usually need a flood risk assessment and may need raised floor levels or flood-resilient construction. Check the government flood map for your address. Source
- River Darent and River Cray flood risk. The River Darent runs through Dartford town centre and up the valley through Hawley, Sutton-at-Hone and Darenth, and the River Cray joins it at the borough's western edge. Land alongside both rivers sits in Flood Zones 2 and 3, so ground-floor extensions there may need a flood risk statement with the application. Source
- Green Belt south of the town. Much of the southern half of the district, including the countryside around Wilmington, Hawley, Darenth, Bean, Southfleet and Longfield, 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
- Conservation areas. Dartford has conservation areas covering parts of the town centre around High Street and Holy Trinity, Wilmington, Stone and several 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
- Listed buildings. Holy Trinity Church and the older buildings of Dartford High Street, the church and cottages at Stone, Wilmington, Darenth and Southfleet, and Sutton-at-Hone contain listed houses and churches. Listed building consent is needed for almost any alteration, internal or external. Source
What Dartford planning officers look for
Dartford officers assess householder applications against the Dartford Local Plan 2021–2037 and its design and amenity policies. On the terraces around the town centre they tend to look for rear extensions that respect the pattern of the row, sit below the neighbour's first-floor sill and clear the 45-degree line to the nearest habitable window, and dormers that are subordinate, set back from the eaves and clad in matching tile or slate. On the inter-war semis of Wilmington and Joydens Wood they look for side extensions set back from the front wall and below the ridge, with a gap kept to the boundary so the pair does not read as a terrace. In Flood Zone 3 north of the A206 they expect a flood risk assessment with any ground-floor extension, and in the Green Belt to the south the disproportionate-additions test applies, usually measured by floorspace and volume. Common refusal reasons are full-width box dormers, side extensions that create a terracing effect, and ground-floor extensions in the flood zone without a flood risk assessment. Outside conservation areas, hip-to-gable and rear-dormer lofts within the 40 m³ or 50 m³ allowance are routinely lawful and we secure a Lawful Development Certificate as proof. Dartford 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.
