What part of a dwelling counts as usable? In terms of heating zones.

All of the dwelling if it's a habitable room.

Maybe a bit more background to your question will help with an answer.

Is this in relation to a Rented property, one for sale by you, one you're buying or one you own?

Sofija Dearden said:

What part of a dwelling counts as usable? In terms of heating zones.

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More info required for full answer. centralheatking

Usable areas for heating zones include any spaces where people regularly spend time and need to be comfortable. This includes living rooms, bedrooms, kitchens, bathrooms, dining areas, home offices, finished basements and attics, hallways, staircases, laundry rooms, and insulated sunrooms or enclosed porches. Non-usable areas, which don't usually need heating, are unfinished basements and attics, garages, storage rooms, and crawl spaces. This helps ensure heating is used efficiently, focusing on the parts of the home that are lived in.