It is sometimes forgotten that ownership of property includes the airspace above as well as the ground beneath, so placing something in that airspace is trespass. In a recent case an advertising hoarding and platform fixed to a wall intruded into the airspace of the neighbouring property. The court had to decide the appropriate level of damages for the trespass. The High Court had awarded damages of 100% of the advertising fees from the hoarding. The Court of Appeal decided that was excessive; the usual approach is to assess the amount which the parties would have negotiated as a reasonable licence fee for placing the hoarding in the airspace.
Source: Stadium Capital Holdings (No.2) Ltd v St. Marylebone Property Company Plc  EWCA Civ 952.