Side extensions in Slough depend on the house. Typical homes here: 1930s semi-detached houses in Cippenham, Langley, Upton, Upton Lea and Wexham Lea; Victorian and Edwardian terraces in Chalvey, around the town centre and along Stoke Road. Designations to check first: Green Belt; Conservation areas; Article 4 direction on HMOs. Applications are decided by Slough Borough Council and the Slough Local Development Framework Core Strategy 2008 (new Local Plan emerging).
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 Slough
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 Slough are listed below, taken from our Slough area page. Each one is a reason to confirm the address on Slough's constraints map before relying on permitted development.
- Green Belt. The countryside to the north and west of the built-up area, around Wexham, Britwell and the Burnham edge, and the Colne Valley to the east around Colnbrook and Poyle is Metropolitan Green Belt. Source
- Conservation areas. Slough has a small number of conservation areas, including Upton Park, the St Mary's area of Langley and Colnbrook village. Source
- Article 4 direction on HMOs. Slough is understood to have a borough-wide Article 4 direction removing permitted development for converting a house into a small house in multiple occupation (C3 to C4), so HMO conversions normally need planning permission. Source
- Jubilee River and Colne Brook flood risk. The Jubilee River flood relief channel runs along the borough's southern edge below Chalvey and Upton, and the Colne Brook and River Colne run through Colnbrook and Poyle; land alongside them sits in Flood Zones 2 and 3, with smaller watercourses such as the Chalvey Ditch adding local flood risk. Source
- Listed buildings. Upton Court and St Laurence's Church in Upton, St Mary's Church and the almshouses in Langley, Baylis House and the older buildings of Colnbrook High Street are among Slough's listed buildings. Source
What Slough planning officers look for
Slough officers assess householder applications against the Core Strategy 2008, its saved policies and the council's residential extensions guidance, with the emerging Local Plan carrying growing weight. On the 1930s semis that dominate Cippenham, Langley and Upton 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, rear extensions that clear the 45-degree line to the neighbour's nearest habitable window, and hipped roofs kept where a gable would unbalance the pair. Common refusal reasons are full-width box dormers, side extensions that create a terracing effect, and extensions that leave too little garden or parking. 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
Slough 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.
- Slough Borough Council: planning: the council's own constraints map, validation checklist and pre-application service.
