Letting a property in class C4 use to a family for any period of time will mean that its use will change to a class C3 dwelling house. You will need to apply for planning permission to change the use of the property ‘back’ to a class C4 HMO if you want to rent it out again to between three and six unrelated individuals.
Renting to less than three unrelated individuals would also mean that the use of the property has changed from class C4 to C3. (Unless, for example, there are other tenancy agreements in place for more occupants but only one or two are currently in residence or the landlord is actively seeking further tenants.)
Another option is to apply for planning permission for mixed C3/C4 use. This will mean that the property can be used in either way. You can apply for permission for a mixed C3/C4 use from 1 November 2011. The fee for this application is £462.