How deep can I extend without planning permission?

Three metres from the original rear wall on a terraced or semi-detached house and four metres on a detached house, under Class A permitted development. Six or eight metres is possible through the larger home extension prior approval, a 42-day process with a £120 fee, outside conservation areas.

Do I need a Lawful Development Certificate if the extension is permitted development?

Not legally, but we recommend one. It is the council's formal confirmation that the extension is lawful, which lenders, buyers and solicitors routinely ask for. The fee is £264 and the decision takes up to 8 weeks.

What if my neighbour has already extended?

Their extension does not change your rights, but it does shape what the council will accept if you need planning permission: officers compare depth and height against the neighbouring extension and apply the 45-degree daylight test from the nearest habitable window.

How long does St Albans take to decide?

The statutory period is 8 weeks from validation for both a householder application and a Lawful Development Certificate, and 42 days for a larger home extension prior approval. Decisions can run past the statutory 8 weeks; allow 8–12 weeks.

Rear Extensions in St Albans: Planning

Rear 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 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' 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. In the Green Belt they check the size of the extension against the original house and resist large outbuildings. 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.

Do I need planning permission for a rear extension in St Albans?