Kensington and Chelsea is the hardest borough in the country to dig under, and it is also the one where most owners want to. Space is scarce, the streets are almost entirely in conservation areas, and pushing the back of the house into the garden is usually not on offer. So people go down.
The borough saw that coming before anybody else did, and wrote rules for it. If you are considering a basement here, those rules decide what you can build long before an architect draws anything.
Why owners here dig rather than extend
Most properties in the borough are restricted by conservation area policy and by what the neighbours can see. A basement adds floor area without changing the front elevation, the roofline or the garden depth, which is why it survives planning where a rear extension does not.
It is also where the rooms go that do not need a window:
- A home cinema
- A gym or a wellness room
- A wine cellar and tasting room
- Plant, storage and utility
- Guest accommodation
Moving those below ground frees the floors above for bedrooms and living space, which is the real gain. The house does not just get bigger, it gets rearranged.
What policy CL7 actually allows
Basements were never simply permitted development here in the way people assume. A basement under the garden, or one adding a lightwell, has always needed planning permission. In January 2015 the borough adopted Policy CL7, the first policy of its kind in the country to limit the extent of a basement rather than just its appearance.
The two limits that matter most are these. A basement may be one storey only, which is what ended double and triple basements in the borough. And it may extend under no more than half the garden, with the unaffected half kept as a single area, usually the end furthest from the house.
The Basements planning guidance that followed in April 2016 adds the detail: the single storey cannot be horizontally subdivided later, floor to ceiling is generally three to four metres, and a small extra allowance is available where there is a swimming pool.
Two more things arrived at the same time. The Code of Construction Practice means a construction traffic management plan is required with the application. And an Article 4 direction confirmed in April 2016 removed permitted development rights for basements entirely under the footprint of the house, which was the only remaining route without an application. Every basement in this borough now needs planning permission.
None of that stops a basement. It decides its size and shape, and it means the feasibility work has to happen first.
Making a basement feel like a room
A below-ground space can feel bright and calm rather than dark, but only if the daylight is designed in from the start. Skylights, lightwells and a subterranean courtyard are the three tools, and all three have to be agreed with planning and coordinated with the structure before excavation.
Ceiling height is the other half of it. Once the waterproofing, the drainage and the ventilation ducts are allowed for, a generous-looking section can lose half a metre. Plan the services with the structure or you will find out too late.
Before you commit
A basement here is a planning question, a structural question and a neighbour question before it is a design question. Get the feasibility right and the rest follows.
Our longer guide sets out what to ask any basement contractor, in ten checks before you appoint, and planning a basement conversion covers the surveys and the sequence.
If you are weighing up a basement in Kensington or Chelsea, tell us about the house. The cost calculator publishes our basement rates, and our basement service sets out what we take responsibility for.
Published 9 February 2025. Written by the NU Projects studio, edited since for house style and accuracy.