What is the 45-degree test?

A line drawn at 45 degrees from the centre of the neighbour's nearest habitable room window. If your first-floor extension crosses it, officers treat it as harming their daylight and will ask for the depth to be reduced or the extension set in from the boundary. We plot it on every two-storey scheme before submission.

Can the ground floor go deeper than the first floor?

Yes, and it usually should. A common approach is a 4 m ground floor with a 3 m first floor set 1 m in from the shared boundary, which keeps the daylight line clear while giving a full-width kitchen below.

Does the roof have to match?

Councils expect the extension roof to follow the form of the main roof, for example a hipped roof on a 1930s semi, and to sit below the existing ridge. A flat-roofed first floor is normally refused on a pitched-roof house.

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.

Two-Storey Extensions in St Albans: Planning

Two-storey 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 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 St Albans

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 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. Common refusal reasons are full-width box dormers, side extensions that create a terracing effect and disproportionate Green Belt extensions.

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 two-storey extension in St Albans?