If you let out part of your home, you could be eligible for 'Rent - a - Room Relief', a valuable tax allowance worth £7,500 a year. So if the rent you charge your lodger is below this figure you won't pay tax on it. The only downside is that you can't claim any further expenses of letting, like a proportion of buildings insurance or mortgage interest against the rents.
For someone who has a property originally designed as a single residence which is temporarily divided into two or more separate residences like the case of a self- contained flat or annex there is an opportunity to claim the £7,500 tax allowance providing the that the separation can be shown to be temporary.
In areas of high housing costs like London and other large cities in the UK where housing availability is at a premium, older properties are often partitioned without permanent structural change.
The tax law provides that 'if a building, or part of a building, designed for permanent use as a single residence is temporarily divided into two or more separate residences, it is still treated as a single residence.'
So it doesn't matter whether the residences are currently separate, if they were designed to form a single residence then they remain so for the purposes of the Rent - a - Room relief meaning that you can claim it assuming that the separation is temporary.