If you look up your property on www.gov.uk/council-tax-bands, you'll see that some nearby ones have an 'improvement indicator' for a change in banding when they're sold.
Strangely, though, some, with the biggest (6 or 8 metre) extensions, though, haven't.
What’s a household item that reminds you of your grandma’s house?
Unused rooms, heat storage and heat transfer
