Although PV cells are most effective in sunlight, they can still produce some power on cloudy days. In new builds, they can be integrated into the design as roof tiles. Retrofitting is also possible if the building will take the load, which will need to be ascertained via a structural survey. They are best installed in a southerly, unshaded area. Unless a building is listed, planning permission isn't normally required for small instillations, although contacting a planner for advice is still recommended.