Section 115 | Parts of a greater whole
From legislation.gov.uk
(1)The fact that a part of a building is suitable for use as a dwelling does not prevent that part from forming part of a larger single dwelling.
(2)The fact that a building or structure that is—
(a)in the garden or grounds of a dwelling, and
(b)occupied or enjoyed with the dwelling,
is itself suitable for use as a single dwelling does not prevent it from being treated (in accordance with section 112(2)) as part of the dwelling.