is interest relief of 25% and 75%*20% only available on but-to-let properties or to other residential properties also? So, for example if a room has been let in a property (with 5 rooms) occupied by the owner himself then is the interest relief available like- interestx1/5×25%(in case a mortgage on the property has been taken out) is this a deductible expense?
Interest relief is only on buy to let property, generally when you take out a buy to let mortgage you have a different terms, for instance interest is higher.
It may be a bit different story, when for instance, you planning to extend your house in purpose of letting only, in that case I could think the relief is available, But our kind tutor can confirm 🙂