Different authorities will have different policies outlining planning permission for under garden basements. It’s vital to know what you can and can’t do and what design is feasible and effective, so make sure you’re familiar with your local policies.
For example, Wandsworth recommends retaining half your garden, whereas Merton explicitly states you must not exceed 50% of any garden space. Also, Kensington and Chelsea limit the extension to half the garden and the depth of a single storey.
There may also be conservation area restrictions. If you have trees in yours or your neighbours garden, you will have to consider the impact of digging into your garden.
When planning your basement project, contact the local authorities to get up to date regulations. If you’re working with a build team, they will also be able to give you guidance and advice on how to build an underground room and what’s feasible.