Side extensions in Windsor and Maidenhead depend on the house. Typical homes here: Victorian and Edwardian terraces and villas in Windsor town, Clewer and central Maidenhead; 1930s semi-detached houses in Cox Green, Furze Platt, Dedworth, Boyn Hill and Pinkneys Green. Designations to check first: Green Belt; Conservation areas; River Thames flood risk. Applications are decided by Royal Borough of Windsor and Maidenhead and the Borough Local Plan 2013–2033 (adopted 2022).
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 Windsor and Maidenhead
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 Windsor and Maidenhead are listed below, taken from our Windsor and Maidenhead area page. Each one is a reason to confirm the address on Windsor and Maidenhead's constraints map before relying on permitted development.
- Green Belt. Most of the Royal Borough outside the built-up areas of Maidenhead, Windsor, Ascot, Sunningdale and the larger villages is Metropolitan Green Belt, including the countryside around Cookham, Bray, Fifield, Old Windsor and Datchet. Source
- Conservation areas. The Royal Borough has conservation areas covering Windsor town centre, Eton, Cookham, Bray, Datchet and Old Windsor among others, with more in Maidenhead, Sunninghill and the villages. Source
- River Thames flood risk. The Thames runs through Cookham, Maidenhead, Bray, Windsor, Eton, Datchet, Old Windsor and Wraysbury, and large areas alongside it sit in Flood Zones 2 and 3. Source
- Listed buildings. Eton High Street, Cookham, Bray, Datchet, Old Windsor and Windsor town centre contain a very high concentration of listed houses, cottages, inns and churches. Source
What Windsor and Maidenhead planning officers look for
Officers at the Royal Borough of Windsor and Maidenhead assess householder applications against the Borough Local Plan 2013–2033 and the council's guidance on residential extensions, with Green Belt policy in the background for most of the borough. On the terraces and villas of Windsor and central Maidenhead they tend to look for rear extensions that respect the pattern of the row, sit below the neighbour's first-floor sill and clear the 45-degree line to the nearest habitable window, and dormers that are subordinate, set back from the eaves and clad in matching slate or tile. On the 1930s semis of Cox Green, Furze Platt and Dedworth they look for side extensions set back from the front wall and below the ridge, with a gap kept to the boundary so the pair does not read as a terrace. Common refusal reasons are disproportionate Green Belt additions, side extensions that create a terracing effect, roof extensions in the Windsor, Eton and Cookham conservation areas, and flood zone extensions without an assessment.
Fees and timelines
Windsor and Maidenhead 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.
- Royal Borough of Windsor and Maidenhead: planning: the council's own constraints map, validation checklist and pre-application service.
