Swindon's design guide puts numbers on the page
Plenty of councils leave neighbour impact to judgement. Swindon wrote its figures down, in the Residential Extensions and Alterations SPD of October 2011. It only applies when you need planning permission, but then your drawings get measured against it.
At the back of the house, the guide caps a single-storey addition at 4m where it sits at or near the boundary, and a two-storey or first-floor one at 3m. Then comes the 45-degree test. For a single-storey build the line is drawn from the centre of the neighbour's nearest habitable room window. Go up a floor and it moves to the quarter point of that window, which is a stricter test. Any wall on the boundary should be solid, with no windows in it.
Going sideways, keep within half the width of the front of the house and leave at least 75mm between the gutter and the boundary. A first-floor side addition must step back at least 900mm from the front wall, with its ridge at least half a metre lower than the main roof. Rows of semis shouldn't end up looking like a terrace.
Two less obvious rules catch people out. The garden left over must stay above a minimum, 30 square metres for a one or two bedroom house, 40 for three bedrooms and 50 for four or more. And back-to-back windows need 18m between them at single storey and 21m at two storeys. A deep two-storey addition on a short Victorian plot can fail on those alone.
Plenty of jobs never reach that guide. Outside a conservation area, national permitted development lets you go 3m past the original back wall of a semi or terrace, or 4m on a detached house, up to 4m high. The neighbour consultation route stretches that to 6m or 8m, with 42 days for the council to respond. Two storeys at the rear are limited to 3m deep, at least 7m from the back boundary.
Ten builds, and where each suits Swindon
Each type opens a longer guide.
Single-storey rear. The usual start on a 1930s semi off Churchward Avenue or a Walcot house from the late 1950s. Inside the permitted limits it needs Building Regulations approval only.
Kitchen. A mono-pitch roof under a bedroom window needs at least 150mm clear below the cill by the council's guide, so a flat roof with lanterns often works better.
Side return. Fits the brick terraces of Rodbourne, Gorse Hill and the Rolleston streets, where a passage runs beside the back addition. Inside the Town Gardens or Prospect Place conservation areas it needs an application.
Open plan. Taking out the back wall needs an engineer's beam. In a post-war Airey or Easiform house at Moredon or Rodbourne Cheney, bring the engineer in first, because the walls aren't brick.
Two-storey. Over the drive of a semi this means a full application, and officers check the 900mm set-back and lower ridge first.
Wrap-around. Its side leg takes the job out of permitted development. Check the leftover garden against the minimum first.
Downstairs bedroom. A level-access room and shower room lets an older relative stay at home. Keep it attached, as the guide treats a granny annexe as a new dwelling, not an outbuilding.
Home office. An attached study runs off the house's heating and broadband. Where a detached garden building needs permission, the guide caps its ridge at 4.2m.
Planning. In any of Swindon's 27 conservation areas, or the North Wessex Downs villages, side additions, two-storey rear ones and cladding all need consent, and the 6m and 8m route is closed.
Cost and value. No price here. Matching stone in Old Town, raised floors near the Ray or deeper footings by a tree all move it.
The Ray, the clay and the trees
The River Ray rises at Wroughton and drains most of the built-up town. The Environment Agency runs a flood warning area for the stretch from Wootton Bassett Road down to the sewage works at Barnfield Road, and a wider alert area for the Ray and Swinbourne across West Swindon. Put your postcode into the flood map. In Flood Zone 2 or 3 an application needs a flood risk assessment, and the standing advice for small extensions asks for floors 600mm above the estimated flood level or flood-resistant construction to that height.
Under most of the town is clay, the Ampthill and Kimmeridge formations, with Old Town on its hill of Portland stone. Your building inspector sets the footing depth once the trench is open, and nearby trees can push it down. In a conservation area any tree with a stem wider than 75mm at 1.5m off the ground is protected, so give notice before roots are cut.
Railway cottages, Old Town and the downland villages
The railway village is the hardest place in Swindon to add space. A 1980 Article 4 direction removes the right to extend or alter the cottages, and most of them are listed, so you need planning permission and listed building consent. In the 1970s the rear yard additions were cleared and replaced with one long brick lean-to along each terrace, holding the kitchens. New work has to respect that line.
Old Town's four conservation areas, from the High Street out to Town Gardens, are mostly Swindon stone or orange-red brick under Welsh slate. Match both, and the roof pitch. South of the town, Liddington's conservation area sits wholly inside the North Wessex Downs, and so does the southern half of Upper Wanborough's, where some roofs are still thatched.
Before paying for drawings, look up approved schemes on your street on the planning register. Building control is an in-house team at Wat Tyler House and needs two days' written notice before you start.
Where these facts come from
- Swindon Borough Council: Residential Extensions and Alterations SPD (2011)
- Swindon Borough Local Plan 2026
- Railway Village Article 4 direction (1980)
- Swindon Railway Conservation Area appraisal (2021)
- Swindon Borough Council: conservation areas
- Swindon Borough Council: protected trees
- Swindon Level 1 Strategic Flood Risk Assessment (2025)
- Environment Agency: River Ray, Swindon flood warning area
- Environment Agency: River Ray and Swinbourne, West Swindon flood alert area
- Environment Agency: flood risk standing advice
- GOV.UK: householder permitted development technical guidance
- Swindon Borough Council: view or comment on a planning application
- Swindon Borough Council: building control contacts
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Questions people in Swindon ask
Our neighbour's kitchen window is close to the fence. Will that stop a two-storey addition?
It may. Swindon's guide draws the 45-degree line from the quarter point of that window for anything above ground floor, rather than the centre used for single-storey work, so upstairs additions near a side window often fail it. A single storey on the same footprint has a better chance.
We're on a corner plot in West Swindon. Can we build out to the side?
Possibly, but Swindon's guide says the new part must stay behind the building line on both streets and not fill the open gap at the corner. Measure where the neighbours' fronts sit on each road before you brief a designer.
We live by the Ray near Wootton Bassett Road. What extra does an application need?
Check the Environment Agency flood map for your address. If the house is in Flood Zone 2 or 3, include a flood risk assessment, following the Agency's standing advice for small extensions on floor levels and flood resistance.
Can we replace the rear lean-to on our railway village cottage?
Not without talking to the council first. The 1980 Article 4 direction and the listing mean planning permission and listed building consent. Email the conservation team with photos before anyone draws plans.