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. 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. 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. 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
- 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. The Jubilee River scheme is understood to protect Maidenhead, Windsor and Eton from most floods, but Datchet, Wraysbury, Old Windsor and parts of Cookham remain at risk. Ground-floor extensions and basements in the flood zones usually need a flood risk assessment and may need raised floor levels. Check the government flood map for your address. 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. Listed building consent is needed for almost any alteration, internal or external, and houses within the setting of a listed neighbour face closer scrutiny on design. 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. In the Green Belt the first question is whether the extension, added to anything built since the house was original, is disproportionate; officers usually measure this by floorspace and volume, and additions beyond roughly a third to a half of the original tend to need a strong case. In the Thames flood zones they expect a flood risk assessment with any ground-floor extension. 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. Outside the designations, 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. The Royal Borough 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.
