Converting an existing residential cellar or basement into a living space is in most cases unlikely to require planning permission as long as it is not a separate unit or unless the usage is significantly changed or a light well is added, which alters the external appearance of the property.

Excavating to create a new basement, a new separate unit of accommodation and/or altering the external appearance of the dwelling, which may involve major works or something as minimal as adding a light well, is likely to require planning permission.

If you live in a listed building will to need consent for internal or external works to the building.

In all circumstances you are advised to contact your Local Planning Authority for guidance on local policy before starting any work.