If this new building is going to be entirely separate with new drainage/water supply etc then I think most councils will expect to tax it. If a more conventional 'granny annexe' connected to the main house, perhaps even with a connecting door then you may get away with out the extra tax.
There are recurring news stories on BBC London of councils clamping down on people who have converted sheds at the end of the garden in to accommodation and are renting them out.
Probably best having a word with the council before going to far with planning the project.
1990ish Dawes 'Bonded Aluminium', 1991 GT Karakoram, 1992 Kona Cindercone, 1993 KHS Montana Comp