Digging a basement in Kensington and Chelsea

Kensington and Chelsea is the hardest borough in the country to dig under. It is also the one where most owners want to. Space is scarce. The streets are almost entirely in conservation areas. 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. You may be considering a basement here. If so, those rules decide what you can build. They do so long before an architect draws anything.

Why owners here dig rather than extend

Most properties in the borough are restricted by conservation area policy. They are also restricted by what the neighbours can see. A basement adds floor area without changing the front elevation. It leaves the roofline and the garden depth alone. That 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. That 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 has always needed planning permission. So has one adding a lightwell. In January 2015 the borough adopted Policy CL7. It was the first policy of its kind in the country. It limits 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. That is what ended double and triple basements in the borough. And it may extend under no more than half the garden. The unaffected half is kept as a single area. This is 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. A small extra allowance is available where there is a swimming pool.

Two more things apply. The Code of Construction Practice governs the site. A construction traffic management plan is required with the application.

None of that stops a basement. It decides its size and shape. 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. That only works if the daylight is designed in from the start. Skylights, lightwells and a subterranean courtyard are the three tools. All three have to be agreed with planning. They also have to be coordinated with the structure before excavation.

Ceiling height is the other half of it. Allow for the waterproofing, the drainage and the ventilation ducts. A generous-looking section can then 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. It is these 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. It does so in ten checks before you appoint. Planning a basement conversion covers the surveys and the sequence.

You may be weighing up a basement in Kensington or Chelsea. If so, tell us about the house. The cost calculator publishes our basement rates. Our basement service sets out what we take responsibility for.