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 St Albans
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 St Albans are listed below, taken from our St Albans area page. Each one is a reason to confirm the address on St Albans's constraints map before relying on permitted development.
- St Albans Conservation Area. The St Albans Conservation Area is one of the largest in the country, covering the city centre, St Michael's, Fishpool Street, Holywell Hill and the streets around the Cathedral, with a high concentration of listed buildings. Roof extensions are not permitted development inside it, and officers apply the character appraisal to extensions, dormers and front alterations. Check the council's constraints map for the exact boundary before relying on permitted development. Source
- Metropolitan Green Belt. Most of the district outside the built-up areas of St Albans, Harpenden and the villages is Metropolitan Green Belt. Extensions to houses in the Green Belt must not be disproportionate to the original dwelling, which officers usually measure against the house as first built, and outbuildings face a stricter test on size and siting. Check the constraints map for the boundary at your address. Source
- Harpenden conservation areas. Harpenden has several conservation areas covering the town centre, the common and surrounding residential streets, and the district also has designated areas in Redbourn, Wheathampstead and other villages. Roof extensions lose permitted development rights inside them. Check the constraints map for boundaries. Source
- River Ver and River Colne flood risk. Land alongside the River Ver through the city, Verulamium Park, Park Street and Redbourn, and the River Colne around London Colney, sits in Flood Zones 2 and 3. Ground-floor extensions and basements there may need a flood risk statement with the application. Source
- Listed buildings. The city centre, St Michael's, Fishpool Street and the village cores of Redbourn and Wheathampstead contain a very high number of listed buildings. Listed building consent is needed for almost any alteration, internal or external. Source
What St Albans planning officers look for
St Albans officers assess householder applications against the saved policies of the 1994 Local Plan Review, the emerging Local Plan 2041 as it gains weight, and the council's supplementary design guidance. On the Edwardian and inter-war semis of Fleetville and Marshalswick they tend to look for two-storey side extensions set back from the front wall and below the main ridge, a gap kept to the side boundary so the pair does not read as a terrace, and rear extensions that pass the 45-degree test to the neighbour's nearest habitable window. In the conservation areas they expect traditional materials, subordinate dormers on rear slopes only and no loss of historic fabric. In the Green Belt they check the size of the extension against the original house and resist large outbuildings. Common refusal reasons are full-width box dormers, side extensions that create a terracing effect and disproportionate Green Belt extensions. Outside conservation areas, 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. St Albans 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.
