Dig in the old city and the archaeologists want to know
Every extension starts with a trench, and in Canterbury that matters more than the drawings. The historic centre has been an Area of Archaeological Importance since 1984. If your house is inside it, the council asks for at least six weeks' notice before any ground works, and it says plainly that this applies even when no planning permission is needed. A permitted development kitchen on a terrace in the centre still triggers it.
Build those six weeks into the programme before you book a builder. Footings, a new drain run and a soakaway all count as digging.
What the planners measure your plans against
There's no district-wide householder design guide here, and the council revoked a long list of older guidance documents from 1 April 2026. Applications are judged against Policy DBE6 of the Local Plan adopted in 2017. It supports extensions that suit the original building in design, size, bulk, height, materials and position, that don't cause unacceptable overlooking or overshadowing next door, and that don't harm the look of the street.
Out in the villages the plan adds a further test. An extension to a house in the countryside should be modest, subservient in scale and in keeping with the existing design. There's no percentage cap. It's a judgement on how the finished house sits in its setting.
For older houses, the 2007 heritage guidance still stands and it's blunt. A new extension shouldn't dominate the main house. Cottages should keep their original form. On a two-storey terrace a rear addition should stay single storey where possible, because two storeys tend to steal light and look into gardens. Materials should match, though weatherboarding sometimes suits better, and a lightweight glazed link can join old to new.
New Dover Road has its own design guide from 2006, and it's the only one in the city that puts numbers on things. Extensions should respect the 45 degree sight lines, both horizontal and vertical, from the houses either side. Gaps of at least 3 to 5 metres to the side boundary should be kept at every level, and flat roofs aren't acceptable.
Clay to the north, river water through the middle
The city centre stands mostly on Thanet Sand, with river alluvium along the Stour. Head north and west towards Blean and the ground turns to London Clay, which the British Geological Survey calls very susceptible to volume change. Tree roots dry it out and it shrinks. On that side of the city, expect building control to ask about nearby trees and the depth of the new footings.
The Great Stour splits into several channels as it runs through town. Kent County Council's flood study lists homes at river flood risk in Westgate, St Stephens, Northgate and Wincheap. In Flood Zone 2 or 3 an extension application needs a flood risk assessment, which for a householder job follows the Environment Agency's standing advice. That advice wants the new floor at least 600 millimetres above the estimated flood level, or flood-resistant construction to the same height.
Which extension suits which Canterbury house
Each type below has a full guide.
Single-storey rear. Outside a conservation area, a semi or terrace can go 3 metres beyond the original rear wall without applying, or 4 metres on a detached house, and 6 or 8 metres with prior approval. In the historic centre, remember the archaeology notice applies whether or not you need permission.
Side return. Wincheap's old terraces are the natural home for one. In a conservation area any side extension needs a planning application, and a party wall notice to the neighbour is likely.
Kitchen. For an older house the council's heritage guidance doesn't favour uPVC or aluminium conservatories or plastic roof sheeting. A simple lean-to in timber or brick with good glazing has a better chance, and usually makes the nicer room.
Open plan. Taking out the old back wall means a steel, and the steel sits on pads that need digging. Have the engineer size them for the ground you actually have, clay or sand, before the builder prices the job.
Two-storey. DBE6's overshadowing test bites hardest here. Where villas have space between them, keep a proper gap to the boundary and a pitched roof. On a terrace, the heritage guidance would rather you stayed at one storey.
Wrap-around. The side leg takes it outside permitted development, so it always needs an application. It suits the interwar semis on the hillsides either side of the city, if there's room down the side.
Ground-floor bedroom. With plenty of bungalows in the district, many families extend one level rather than add a staircase. Wide doors and a level threshold cost little if they're drawn in from the start.
Home office. An attached study shares the house's heating and broadband. A separate cabin may dodge a planning application, but it still needs foundations, so in the old centre the archaeology notice can still apply.
Planning. In a conservation area there are no permitted side extensions, no permitted rear extensions over one storey and no cladding. Where an Article 4 direction applies, as on many unlisted houses in the city conservation area, front alterations need consent too. Check the interactive planning maps first.
Cost and value. We don't print prices. An archaeological watching brief, deep footings near trees on clay or a raised floor in a flood zone can each move the figure a long way. Get builders round to see the plot.
Where these facts come from
- Planning Committee agenda, 6 February 2024, quoting Policy DBE6 (Canterbury City Council)
- Canterbury District Local Plan 2017 (Canterbury City Council)
- Heritage, Archaeology and Conservation guidance, 2007 (Canterbury City Council)
- New Dover Road planning design guide, 2006 (Canterbury City Council)
- Revocation of supplementary planning documents, April 2026 (Canterbury City Council)
- Areas of archaeological importance (Canterbury City Council)
- Clay shrink-swell (British Geological Survey)
- Flood risk to communities in Canterbury (Kent County Council)
- Flood risk assessment standing advice (GOV.UK)
- Permitted development for householders, technical guidance (GOV.UK)
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Questions people in Canterbury ask
We live off New Dover Road. How close to the side boundary can we build?
The council's 2006 design guide for the road asks for side separations of at least 3 to 5 metres, depending on the location, kept at all levels. It also wants the 45 degree sight lines from neighbouring houses respected. A two-storey side addition that closes the gap is unlikely to get through.
We're on the north side of the city, towards Blean. Will the clay affect the foundations?
It may. The ground north and west of the city is London Clay, which shrinks and swells as it dries and wets, and trees make it worse. Building control will check the depth on site, and a big tree near the new wall can push it deeper.
Do I need a flood risk assessment for an extension near the Stour in Westgate?
If the house is in Flood Zone 2 or 3, yes. For a householder extension it's usually a short form following the Environment Agency's standing advice, showing the floor level or flood-resistant construction you plan to use. Look up the flood map for planning before you pay for drawings.
Does the Stodmarsh water rule apply to my extension?
The council's guidance aims it at new housing and overnight accommodation, and doesn't mention householder extensions. An annexe that would be lived in as a separate home is a different matter, so ask the council before you apply.