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. Extensions to houses in the Green Belt must not be disproportionate to the original dwelling, and outbuildings face a stricter test on size and siting. Check the council's constraints map for whether your plot is inside or outside the boundary. Source
- Conservation areas. Slough has a small number of conservation areas, including Upton Park, the St Mary's area of Langley and Colnbrook village. Roof extensions are not permitted development inside them, and officers apply the character appraisal to extensions and dormers. Check the council's constraints map for the exact boundaries before relying on permitted development. 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. Check the council's constraints map for any further directions before relying on permitted development or prior approval. 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. Ground-floor extensions and basements there may need a flood risk statement with the application. Check the government flood map for your address. 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. Listed building consent is needed for almost any alteration, internal or external. 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. On the Chalvey terraces they look for rear extensions that respect the pattern of the row and dormers that are subordinate and set back from the eaves. Parking and the loss of front gardens to hardstanding come up more often here than in inner London, and HMO conversions are scrutinised closely under the Article 4 direction. 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. Slough 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.
