Your question does not deal with what is legal or illegal regarding whether or not your potential landlord will allow small heaters in each room or not. Rather it is a contractual requirement that the potential landlord wants.Possibly the landlord had a bad experience with small space heaters causing a fire in the past and that is why he or she does not want the small electric heaters in the indididual rooms. I suggest that you sit down with the potential landlord and ask why he or she does not want the heaters in individual rooms.