Above 3kW and it is a job for a qualified electrician. Below that, wall mounted units can be fixed by anyone with the ability to drill a few holes and put a plug in.
A 3-pin domestic socket is not designed for continuous high-power usage and so most manufacturers recommend the use of 13A three-pin sockets for 3kW heaters only for light and infrequent use. A heater should be hard wired into a 20 or 30A fused spur or via a suitably rated commando socket when 2.5kW or more power is drawn for longer periods of time. Running a domestic 13A plug and socket this close to its maximum 3kW capacity puts a strain on your circuit, the cables, and the socket, with the potential to cause serious problems, such as overheating and in worst case scenarios, fires. Always consult your electrician on the exact specification of your electrial circuit.