You'll need around five square metres of roof space which faces east to west through south and receives direct sunlight for the main part of the day.
The panels don't have to be mounted on a roof: they can be fixed to a frame on a flat roof or hanging from a wall.
If a dedicated solar cylinder is not already installed then you will usually need to replace the existing cylinder, or add a dedicated cylinder with a solar heating coil.
Most conventional boiler and hot water cylinder systems are compatible with solar water heating. But if your boiler is a combination boiler (combi) and you don't currently have a hot water tank, a solar hot water system may not be compatible.