There's no restriction, assuming that the airspace is uncontrolled, or you have permission to enter it, if it is controlled.

There are some restrictions above residential areas if they're regarded as "congested". Generally, you don't fly below 1,000 feet and you must be able to glide clear (to carry out your forced landing in a field) if you have an engine failure, if flying a single engine aircraft.

Police aircraft are exempted from many [/quote]restrictions.