Short answer: sometimes. It depends on what you’re doing, and — this being London — on which borough you’re doing it in.
Converting a cellar you already have
If your house already has a cellar and you want to turn it into usable space without changing the outside of the building, there’s a fair chance you won’t need planning permission at all. Converting existing space is generally treated as permitted development, the same way a loft conversion often is.
The moment the work changes the external appearance — a lightwell at the front, an external stair, a new entrance — you’re usually into planning territory. And if you live in a conservation area or a listed building, assume nothing and check first.
Digging a new basement
Creating a new basement, or making an existing one bigger or deeper, is a different story. That’s engineering work under (and sometimes beyond) your house, and in almost every case it needs a full planning application.
Several boroughs have been dealing with basement applications for years and now have dedicated basement policies. Some limit how deep you can go, some restrict how much of the garden you can build under, and most will want a construction method statement with the application — a document explaining how the dig will be done without upsetting the street. We’ve been through this process plenty of times, and it’s one of the reasons projects like our Earl’s Court double-storey basement got built without drama.
The Party Wall Act — the bit people forget
Planning permission is one thing. If you share a wall with a neighbour — and in most of London you share two — the Party Wall etc. Act 1996 applies whether planning was needed or not.
Underpinning a shared wall means serving formal notice on the neighbours, usually appointing a surveyor, and agreeing a party wall award before work starts. It sounds heavier than it is. Done early it’s a formality; left late it’s the thing that stalls a project for months. Our advice is simple: start the party wall conversation as soon as your design is settled.
Building regulations apply either way
Whatever the planning situation, the work itself must comply with building regulations — structure, damp-proofing, ventilation, escape routes, ceiling heights. Every excavation and underpinning scheme we build is inspected stage by stage and signed off, pin by pin.
Where to start
Don’t start with the council — start with a feasibility conversation. We deal with basement planning permission in London week in, week out, so we’ll look at the property, tell you honestly what the borough is likely to accept, and point you to the right planning route before you spend money on drawings. Book a free site visit and bring your questions.