Side extensions in St Albans depend on the house. Typical homes here: Georgian and Victorian townhouses and cottages in the St Albans city centre conservation area, including Fishpool Street, Holywell Hill and St Michael's; Edwardian and inter-war semi-detached houses and terraces in Fleetville, Bernards Heath and Marshalswick. Designations to check first: St Albans Conservation Area; Metropolitan Green Belt; Harpenden conservation areas. Applications are decided by St Albans City and District Council and the St Albans District Local Plan Review 1994 (saved policies) and emerging St Albans Local Plan 2041.
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 St Albans
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 St Albans are listed below, taken from our St Albans area page. Each one is a reason to confirm the address on St Albans' 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. Source
- Metropolitan Green Belt. Most of the district outside the built-up areas of St Albans, Harpenden and the villages is Metropolitan Green Belt. 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. 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. 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. Source
What St Albans planning officers look for
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. 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.
Fees and timelines
St Albans 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
- 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.
- St Albans City and District Council: planning: the council's own constraints map, validation checklist and pre-application service.
