Can't see any reason why this would not be allowed under planning rules unless it was converted to flats recently. If it was then the permission granted is likely to be based on the approved plans so you could not then add extra bedrooms without getting it approved again. Best to get a lawful development certificate from the council before you start.
Also you might have issues with building regs as loft rooms will need windows for ventilation
Also you might have issues with building regs as loft rooms will need windows for ventilation