Subordinate, not over-dominant
The Royal Borough doesn't hand you a maximum depth. Its Borough Wide Design Guide, adopted in June 2020, sets a test of character instead. Principle 10.1 expects an extension to be subordinate and to respond to the form, scale, architectural style and materials of the original building, and it says plainly that over-dominant or out of keeping schemes will be resisted.
Officers pair that with Borough Local Plan policy QP3, which asks that a scheme has no unacceptable effect on the people next door. So the questions at the planning desk are about bulk, light and outlook, judged house by house. Draw the neighbours' windows on your plans before anyone draws your new kitchen.
Subordinate looks different on each street. A side addition to a yellow stock brick cottage off Ray Street wants to sit low and quiet. On the red brick and tile-hung houses of Boyn Hill, the planners will notice if the new roof ignores the old one. The 1930s semis of All Saints Avenue have their own clay tile and chimney pattern to follow.
Matching the job to the house
How each kind of extension fits Maidenhead's streets. Each one links to a longer guide.
Single-storey rear. Outside a conservation area you can usually add 3 metres at the back (4 on a detached house) with no application. The national prior approval route stretches that to 6 or 8 metres, but not on conservation area land.
Kitchen. If the new floor area and patio add hard surfaces, the council's validation page asks you to explain where the rainwater goes, so plan soakaways or storage early.
Side return. Suits Victorian terraces with a narrow passage beside the back addition. It counts as a side extension, so inside one of the seven conservation areas it needs planning permission.
Open plan. Knocking through the back of a brick terrace needs steel. Get the engineer's calculations before builders price it.
Wrap-around. Part side, part rear, so on Ray Park Avenue or around All Saints church it is a full application from the start. Elsewhere the side element is limited to one storey and half the width of the original house.
Two storey. This is where Principle 10.1 bites. A two-storey side addition to a semi needs to read as secondary to the house, and in a conservation area a two-storey rear addition loses its permitted development status altogether.
Downstairs bedroom. A bedroom and shower room for an older parent can follow the ordinary rules. Near the river, ask your designer about floor levels, because a flood risk assessment will raise them anyway.
Home office. Attached to the house, an office is heated, wired and on the same insurance. A cabin at the end of a short terrace garden is none of those by default, and in a conservation area its siting is controlled more tightly.
Planning. Check the conservation area map and the flood map first. On the new estates of South West Maidenhead, read the original consent for conditions too.
Cost. We don't put a price on it. A flood risk assessment, a tree survey, steel and matching clay tile all move a Maidenhead quote. Ask three builders to price the same drawings.
Paperwork for homes near the river
The Environment Agency runs a flood warning area for the Thames at Maidenhead, Bray, Dorney, Windsor and Eton, plus a separate one for moorings and the properties closest to the water. If your plot is in Flood Zone 2 or 3, the council's validation guidance says the application should come with a flood risk assessment. For a householder extension that follows the Agency's standing advice, so it is a short document, not a study.
Leave it out and you can lose. Policy NR1 of the Borough Local Plan asks for an assessment wherever land is liable to flooding, and Maidenhead householders have had appeals dismissed for that reason alone. Others won once they showed the extension wouldn't block flood water or eat into the floodplain's storage.
Groundwater is the quieter problem. Thames Water's plan for the town records exceptionally high groundwater in 2023 and 2024 over the chalk, with London Clay and the Lambeth Group also in the catchment and river gravels near the Thames. Building control will want to see the trenches before concrete goes in.
Conservation areas, trees and the Green Belt
The town's seven conservation areas are the Town Centre, Maidenhead Riverside, All Saints at Boyn Hill, Altwood Road, Castle Hill, Furze Platt Triangle and Pinkneys Green. Inside them there is no permitted development for a side extension, a rear extension of more than one storey, or cladding. We found no Article 4 direction removing the ordinary extension rights on the town's streets, but ask the council before relying on that.
Trees matter more than people expect. If your site has trees in a conservation area or under a preservation order, the council wants tree information with the application, and a BS 5837 survey with a protection plan where a big tree could be affected.
Most of the countryside around the town is Green Belt. Policy QP5 points to national policy rather than setting its own percentage, and extensions there are judged on whether they are disproportionate to the original house. You'll need plans and elevations of what stands there now.
Where these facts come from
- RBWM: Borough Wide Design Guide SPD (2020)
- RBWM Planning Committee report quoting Design Guide Principle 10.1 (2021)
- RBWM: what to submit with your planning application
- RBWM: planning appeals report, NR1 flood risk (2023)
- Environment Agency: River Thames at Maidenhead to Windsor and Eton flood warning area
- Environment Agency: properties closest to the Thames, Maidenhead to Eton
- RBWM: conservation areas
- Thames Water: Maidenhead groundwater management plan
- Maidenhead Neighbourhood Plan: non-designated heritage assets
- GOV.UK: Permitted development rights for householders, technical guidance
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Questions people in Maidenhead ask
We live close to the Thames near Boulters Lock. Do we need a flood risk assessment for a small kitchen extension?
If the plot is in Flood Zone 2 or 3, yes, even for a small one. The council's guidance says householder extensions follow the Environment Agency's standing advice. Check your address on the government flood map first.
Can we use the 6 metre prior approval route on a semi in Furze Platt?
Only if the house is outside the Furze Platt Triangle conservation area. On conservation area land that larger route isn't available, so anything beyond 3 metres needs a full householder application.
Does RBWM have a set depth limit for rear extensions?
Its 2020 design guide works on character instead, asking that extensions are subordinate and not over-dominant. Officers then judge the effect on the neighbours under policy QP3.
There's an old oak by our back fence in Pinkneys Green. Does that change anything?
Probably. Pinkneys Green is a conservation area, and trees there or under a preservation order mean the council expects tree information with the application. A BS 5837 survey is the usual answer.