Two sets of numbers, and which one applies
Slough homeowners get caught between two rulebooks. Build under permitted development and the national limits apply: 3 metres out from the original back wall on a semi or terrace, 4 on a detached house, no higher than 4 metres. Go bigger, or build in a conservation area, and you're into a planning application judged against the council's Residential Extensions Guidelines, adopted in January 2010.
Those guidelines are more precise than most. A single-storey rear addition can go 3.65 metres deep on a terrace and 4.25 metres on a semi or detached house, measured from the original rear wall. Flat roofs should stay under 3 metres high. Two storeys at the back are capped at 3.3 metres deep as a general rule, and on a mid-terrace house the guide says they generally can't be done at all.
One trap is worth knowing. Slough adds old and new together, so a house that already has a 2-metre lean-to doesn't get a fresh 3.65 metres on top. The guide calls that leap-frogging and won't allow it.
Garden left over, cars kept on the drive
Two tests decide more Slough applications than the extension's looks do. The first is the garden you keep. A two or three-bedroom house should keep a rear garden at least 9 metres deep (or 50 square metres). Four bedrooms or more needs 15 metres (or 100 square metres).
The second is parking. The council expects two spaces off the road for a two or three-bedroom house and three for four bedrooms or more, though that last figure can drop to two near good public transport. Turn a garage into a room, or add a bedroom while losing a space, and the guide says no unless the standard is still met. Bring your drive measurements to the first meeting with a designer.
Choosing the right type for your house
Single-storey rear. The obvious first move on most Slough semis and terraces, and often permitted development. Bigger ones up to 6 metres (8 on a detached) can go through prior approval, where neighbours are notified and the council has 42 days to respond. That route is closed in the five conservation areas.
Kitchen. If the new kitchen needs a soakaway for the roof water, the guide wants it at least 5 metres from the extension. On a short plot that decides the drainage design, so sort it before the builder prices it.
Open plan. Knocking the old back wall through means steel, and Slough Building Control sends an inspection plan listing the stages its surveyor needs to see. Get the engineer's calculations in hand first.
Side return. Single-storey side work here can't be taller than 4 metres, and the guide expects a pitched roof and no windows in the flank wall facing next door. Inside a conservation area it needs permission.
Two storey. Beside a semi neighbour, the first floor must sit at least 1 metre off the side boundary, be set back a metre from the front wall, and have its ridge half a metre below the main roof. If next door has already built up to the line, the gap between the two first floors should be 2 metres. Already converted the loft from hip to gable? A side addition on top is unlikely to be approved.
Wrap-around. Because part of it runs past a side wall, it falls outside permitted development across Slough's conservation areas. Where the plot is a corner one, the guide wants walls parallel to the house, not wedged to fill a splayed garden.
Downstairs bedroom. The local plan lets the council relax its extension criteria for a family member's medical needs. A granny annexe must be built onto the house, with no separate front door, and is meant for an elderly relative or dependant.
Home office. A cabin at the end of the garden counts against the depth you keep, and the guide limits outbuildings to one storey. Attached to the house, an office is simply another room.
Planning. Householder permitted development rights don't cover flats or maisonettes, which matters in a town with so many flats. For a house, a Lawful Development Certificate from the council settles any doubt before you build.
Cost. We don't quote figures. What moves a Slough price is local: a flood risk assessment near the brooks, natural slate in a conservation area, or steel for a deep open-plan room.
Brooks, groundwater and the ground underneath
Roughly 15% of the borough lies in Flood Zone 2 or 3, according to the council's flood risk study. The zones follow the small watercourses that run north to south through town, including Salt Hill Stream, Chalvey Ditches, Datchet Common Brook and the Colne Brook. An extension in either zone needs a flood risk assessment sent in with any planning application.
Groundwater is the other local issue. The same study says almost half the borough could see groundwater flooding at surface, and that the whole of Colnbrook and Poyle is prone to it. Thinking of a basement under the extension? Talk to an engineer before anything else.
Below the gravel and silt, Slough's bedrock is London Clay. The British Geological Survey advises that foundations on shrinkable clay be designed for it, and that big trees near a house need expert advice before anyone removes or plants them. Ask your building control surveyor to look at the trench before the concrete goes in.
Where these facts come from
- Slough Borough Council: Residential Extensions Guidelines SPD 2010
- Slough Borough Council: Local Plan 2004 saved policies
- Slough Borough Council: Strategic Flood Risk Assessment 2021
- Slough Borough Council: permitted development
- Slough Borough Council: conservation areas
- Slough Borough Council: building control and your home
- GOV.UK: Permitted development rights for householders, technical guidance
- British Geological Survey: shrink-swell
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Questions people in Slough ask
We live on part of Upton Court Road. Is a two-storey side extension possible?
Harder than elsewhere. The 2004 Local Plan named parts of Burnham Lane, Langley Road, London Road and Upton Court Road as Residential Areas of Exceptional Character, and the extensions guide says two-storey side additions there will generally be refused. Ask the planners before paying for drawings.
Can I build a two-storey rear extension on my mid-terrace in Slough?
Usually not. The council's guide says mid-terrace houses generally can't take first-floor or two-storey rear additions, because of the effect on the houses either side. A single-storey one is the realistic option.
Our porch plans got questioned. How big can a porch be here?
The guide caps front additions at 2 metres deep on a detached house, 1.75 on a semi and 1.5 on a terrace. It also turns down flat roofs and decorative columns on porches.
Does my extension need a planning application if I'm in Colnbrook?
Check two things. Much of the old village is a conservation area, so side additions and two-storey rear extensions need permission. Parts of Colnbrook also sit in the functional floodplain of the Colne, which brings flood risk into any application.