Is there a size limit for a garden building on a Green Belt plot in Windsor and Maidenhead?

Not a fixed one. In an April 2026 appeal decision on a plot in Cookham, the Inspector recorded that neither Local Plan policy QP5 nor the Framework defines a disproportionate addition, so it is a matter of planning judgment. A detached garage several metres from the house was treated as a separate building. Where a garden room needs permission in the Green Belt, we measure it against the plot and explain why its size is proportionate.

Do I need a flood risk assessment for a garden room in a Thames flood zone?

The Environment Agency standing advice sets out flood risk assessment for minor extensions of up to 250 square metres in Flood Zone 2 or 3, including floor levels 600 mm above the estimated flood level. It does not mention outbuildings. We confirm with the council whether it will apply that advice to a detached garden room, and look at your address on the government flood map first.

Does an Article 4 direction stop me building a garden room in the Royal Borough?

The directions on the council's published list do not target garden outbuildings, but some do affect gardens: gates, fences and walls in parts of Wraysbury, Bisham and Waltham St Lawrence, and moveable structures and caravans used on land in Wraysbury, Horton and Datchet. We check your address before relying on permitted development.

Does a garden office need Building Regulations approval?

Not if it is under 15 m², or under 30 m² with no sleeping accommodation and either built substantially of non-combustible materials or set at least 1 m from every boundary. Electrics still need a certificate from a registered electrician.

Can I put a bathroom in it?

A WC or shower does not by itself take an outbuilding out of permitted development, as long as the building remains incidental to the house and is not used for sleeping. Drainage may need building control notification.

Why does 2.5 m matter so much?

Most London gardens are narrow, so the outbuilding ends up within 2 m of a boundary, where the overall height limit is 2.5 m from the highest adjacent ground. That governs the roof build-up and floor level, which we design to the millimetre.

How long does Windsor and Maidenhead 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.

Garden Rooms and Outbuildings in Windsor and Maidenhead

Garden rooms in the Royal Borough go behind three kinds of house. The 1930s semis of Cox Green, Furze Platt, Dedworth and Boyn Hill have long, regular back gardens where an office or studio is the obvious use of the bottom third. The Victorian and Edwardian terraces and villas of Windsor, Clewer and central Maidenhead have shorter, tighter plots, so height and distance from the boundary decide the design. And the large detached houses of Ascot, Sunningdale, Bray and Fifield sit on generous grounds, where the question is less space than whether a detached building is appropriate on the plot at all.

The permitted development rules for garden rooms and outbuildings

An outbuilding is permitted development on a house if it is single storey, no higher than 2.5 m where it comes within 2 m of a boundary, otherwise no higher than 4 m with a dual-pitched roof or 3 m with any other roof, sits behind the front wall of the house, covers together with other outbuildings no more than half the land around the original house, and is used for purposes incidental to the house rather than as a bedroom or self-contained flat.

Anything used for sleeping or as a separate dwelling needs planning permission regardless of size. Flats have no permitted development rights for outbuildings, and listed buildings need listed building consent.

What changes in Windsor and Maidenhead

In a conservation area an outbuilding is still permitted development behind the house, but not to the side of it, and the same height limits apply.

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

How the Green Belt treats a detached garden building in Windsor and Maidenhead

Borough Local Plan policy QP5 applies national Green Belt policy to the rural areas of the borough, which include Green Belt land and the settlements washed over by it. It says the Green Belt will be protected against inappropriate development, and that permission will not be granted for it unless very special circumstances are shown. That is how a Planning Inspector summarised the policy in an April 2026 appeal decision on a plot in Cookham.

The same decision is useful for garden buildings. The Inspector treated a garage standing several metres from the house as a separate building, judged on its own, and measured any enlargement against the building as first built. The decision records that neither QP5 nor the Framework defines what counts as a disproportionate addition, so it is a matter of planning judgment. In that case an extension that took the garage footprint up by over 50% and the floorspace by over 100%, with a higher roof, was disproportionate, and its effect on openness weighed against it. The appeal was dismissed.

So there is no fixed percentage to design to. The Planning Portal's list of permitted development conditions for outbuildings turns on height, position, coverage and use, and does not mention the Green Belt. Where a garden room fits those conditions we prove it with a Lawful Development Certificate. Where it does not, and the plot is in the Green Belt, we measure the building, set out the case for its size and keep it modest, because the council and an Inspector will judge it on the plot.

Thames flood zones and Article 4 directions for a garden room

For plots along the Thames, the Environment Agency standing advice defines a minor extension as a householder or non-domestic extension of no more than 250 square metres, and says to use that advice to complete the assessment for a minor extension in Flood Zone 2 or 3. It recommends floor levels at least 600 mm above the estimated flood level (300 mm where the level is known with high certainty), and extra flood resistance and resilience measures where floors cannot be raised. The page does not mention outbuildings, so we ask the council whether it will treat a garden room as a minor extension before we draw the floor level.

The council's permitted development page lists the Article 4 directions that remove rights in the borough. It does not list one aimed at garden outbuildings. It does list directions on gates, fences and walls at Wraysbury Embankment, Bisham and Waltham St Lawrence, one on moveable structures and caravans used on land in the parishes of Wraysbury, Horton and Datchet, and one covering most minor development to front elevations in the Bray Conservation Area. The page also says a direction would normally come to light when you buy a property, so we check your address against the council's current list before relying on permitted development.

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. 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. 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. The Royal Borough offers a paid pre-application service.

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

Planning Inspectorate: Appeal decision APP/T0355/D/25/3374768, The Coppice, Cookham (2 April 2026)
Royal Borough of Windsor and Maidenhead: Permitted development rights and Article 4 directions
GOV.UK: Flood risk assessment standing advice
Planning Portal: Outbuildings, planning permission
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.