Garage conversion to annexe — council wants full planning, I thought it was PD?

We have a detached double garage (built 1998, so not original to the house) and want to convert it into a granny annexe for my mother. No external changes, just insulation, plumbing, and a small kitchenette.

I assumed this was permitted development as it's a change of use within the curtilage. But the council says because it will be used as independent accommodation, it requires full planning permission.

Is this correct? I've read conflicting advice online. Some say if it's ancillary to the main dwelling it's fine, others say any separate kitchen triggers planning.

The council is correct on this one. PD rights for outbuildings specifically exclude use as independent living accommodation. If it has a kitchen and sleeping facilities, it's a separate dwelling unit and requires full planning permission.

The "ancillary" argument only works if there's no kitchenette — a bedroom and bathroom in a garage is fine, but add a hob/oven and it becomes a dwelling.

That makes sense. We'll go for full planning then. Might as well do it properly.