In effect, the owner of a building with windows that have received natural daylight for 20 years or more is entitled to forbid any construction or other obstruction that would deprive him or her of that illumination. Neighbours cannot build anything that would block the light without permission. The owner may build more or larger windows but cannot enlarge their new windows before the new period of 20 years has expired. It is also possible for a right to light to exist if granted expressly by deed, or granted implicitly, for example under the rule in "Wheeldon v. Burrows" (1879).