The planning regime covering the creation of living space in basements is evolving and under review.
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 which involves major works, a new separate unit of accommodation and/or alters the external appearance of the house, such as adding a light well, is likely to require planning permission.
If you live in a listed building you are likely to need consent for internal or external work.
In all circumstances you are advised to contact your Local Planning Authority for guidance on local policy before starting any work.
Learn more on the Planning Portal.
Find a planning consultant.
Getting your job done safely.