Three metres, one storey, nothing to the side
Start with what you can still do. Every house inside the city sits on protected land under the national planning rules, because the whole city is a World Heritage Site. That reaches the interwar semis and postwar estates too.
One right survives. On an unlisted house you can add a single-storey rear extension up to 3 metres deep, or 4 metres on a detached house, no more than 4 metres high, without a planning application.
Past that, you apply. The larger home extension scheme, which lets owners in other towns go out 6 or 8 metres after neighbours are consulted, is closed to houses on protected land. So is any side extension, any rear addition of more than one storey, and cladding on any part of the house. The cladding rule catches people out. Timber boards on a new kitchen wall mean an application.
A listed house is a different conversation. The Bath Preservation Trust counts over 5,000 listed buildings in the city, and for those listed building consent comes first, whatever the size. If an unlisted house fits the 3 metre limit, a lawful development certificate puts that in writing.
Planners read the back of the house like the front
Bath has no householder design guide with a 45 degree rule or a maximum rear depth. Applications are judged against the local plan's amenity and heritage policies and against the character appraisal for your part of the city. Read yours first.
The Bear Flat and Oldfield Park appraisal says the rear of a building "may well be of equal importance as that of its frontage". It names oversized extensions among the changes wearing the area down, and it warns against additions that close the gaps between houses and cut off longer views. Widcombe's appraisal points out that its northern slopes face across the city. A flat roof halfway down a hillside garden is in plain view from the streets above, and the appraisal cites a green roof used to soften exactly that.
The Trust comments on a lot of applications, and it isn't against modern design. For a listed Georgian terrace it backed a contemporary single-storey addition kept low behind the garden wall, saying the large glazed doors made it look lighter. Elsewhere it called a rear addition climbing up a listed house "quite excessive" and said a poor extension next door was no excuse.
Mind the trees. In a conservation area any tree with a trunk 75mm or more across is protected, and work to one that isn't covered by a planning permission needs six weeks' notice to the council.
Hillsides, mines and floodwater
Bath is ringed by hills, and its slopes matter once you start digging. The British Geological Survey warns that old landslide deposits around Bath can be set moving again by excavation, extra load or a change in groundwater. A rear extension cut into a sloping garden can do all three. It also flags gulls, open or rubble-filled fissures in the rock that leave a footing badly supported. Pay for a site investigation before the foundations are drawn.
In Combe Down there's the legacy of the Bath stone mines. The shallow workings were filled with foamed concrete, and the council keeps a project team for questions about them. The BGS also says radon protection is required in extensions in parts of the district, so ask building control whether your street is one.
Near the Avon the issue is water. About 2.8 per cent of the city is in Flood Zone 3a and 2.4 per cent in Zone 2, mostly around Grosvenor, Kingsmead, Lower Weston and Locksbrook. An application there needs a flood risk assessment. The Environment Agency's standing advice asks for a floor 300mm to 600mm above the estimated flood level, or flood-resilient construction.
How each type suits a Bath house
Ordered by how often each fits the city's rules.
Single-storey rear. The one addition that can still go ahead without an application, on an unlisted house within the 3 or 4 metre limit. Behind an Edwardian terrace in Poets Corner that's room for a decent kitchen. Use matching materials, not cladding.
Kitchen. On the slopes of Widcombe and Lyncombe the garden often falls away from the back door. Glazing and floor levels decide whether the kitchen looks at the view or a retaining wall.
Open plan. The rear walls of older Bath houses are often coursed rubble stone, so opening one up needs a properly designed beam and padstones. A listed house needs consent for the knock-through itself.
Side return. Victorian side passages were built for coal deliveries. Filling one counts as a side extension, so it needs planning permission anywhere in the city, and expect a party wall notice too.
Wrap-around. Part side, part rear, so always an application. It suits the semis of Weston and Newbridge, where officers watch the side gap.
Two-storey. Never permitted development in Bath. The questions will be the neighbours' light and outlook, the gap to the boundary and how the new roof meets the old. Hillside backs are judged almost as fronts.
Home office. Attaching it keeps the rules simpler. A separate garden building in a World Heritage Site is capped at 10 square metres if it's more than 20 metres from the house, and none can sit beside the house without permission.
Ground-floor bedroom. Level access is the hard part on a steep plot. Raising the new floor to meet the house helps, and in a flood zone does double duty.
Planning. Assume an application unless it's single storey, at the back, on an unlisted house and inside the limits. The council recommends a planning agent in all cases.
Cost. We don't quote a Bath figure. Stone walls, consent drawings and sloping gardens all move the price. Three local quotes show by how much.
Where these facts come from
- Permitted development rights for householders, technical guidance (GOV.UK)
- Check if you need planning permission (B&NES)
- Bear Flat and Oldfield Park character appraisal (2020)
- Widcombe and Kennet and Avon Canal character appraisal (2020)
- Existing Dwellings in the Green Belt SPD (2008)
- Geology of the Bath area: geological hazards (BGS)
- Level 2 Strategic Flood Risk Assessment for Bath (2009)
- Flood risk assessment: standing advice (GOV.UK)
- Trees in conservation areas (B&NES)
- Bath Preservation Trust: listed buildings
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Questions people in Bath ask
Can I use the 6 metre prior approval route on my semi?
No. That scheme only applies to houses outside protected land, and the whole city is protected. Anything over 3 metres (4 detached) needs a full application.
We live in Combe Down. Should we worry about the old mines?
Ask before you dig. The shallow workings were filled with foamed concrete, but the BGS still links subsidence to the freestone mines there. A site investigation and a word with the council's Combe Down Stone Mines Project Team will tell you what the footings need.
Can the builder take down part of our front wall to get machinery in?
Not without asking. In the Bath Conservation Area, demolishing a wall, fence, gate pillar or railing on or next to the highway needs planning permission.
Our house is on the edge of the city near open fields. Is there a size cap?
If it's in the Green Belt, which covers just over 70 per cent of the district, the council's 2008 guidance says an extension adding about a third to the original house is more likely to be accepted. Check the policies map first.