A few years ago I built an extension to my garage to form a granny flat at the bottom of my garden. Although all utilities are on one bill the local council have registered the flat as a separate address and levy council tax on it, even though the only way of reaching it is through my garden.
The council can charge council tax on any “self contained unit”, which is described by the Valuation Office as “a building, or part of a building, which has been constructed or adapted for use as separate living accommodation”. The annexe will be given its own council tax band, but thanks to a recent change in the law, from 1st April 2014 a relative in the annexe is entitled to a 50% discount. You’re entitled to your money back if you’ve overpaid in the last three years.