In article .com,
writes:
Simon wrote:

Would I need planning permission for this?

Normally no, but you need to contact your local council as there may be
specific planning conditions on your development which require you yo
keep the garage as a garage to reduce on street parking.

I think that's the case in most urban areas nowadays.
There's normally a requirement for some number of off-
street parking places depending on number of bedrooms
and the location, and a garage counts for this purpose.

My neighbour had permission refused to convert their
garage because that wouldn't leave them with enough
off-street parking for the number of bedrooms they have.
(The fact the garage is never used for parking is not
relevant.)

--
Andrew Gabriel