Will I have to pay a second council tax bill?

If the unit is assessed as self-contained, the Valuation Office Agency will normally give it its own band. That decision rests on the physical characteristics of the space rather than your planning consent. A 50 per cent annexe discount applies where the unit is used as part of the main home or lived in by a relative, and an exemption may apply where a dependent relative occupies it. Discounts and exemptions are applied for through your council, not the VOA.