Claremont Square is the street in Islington, London, where they filmed the exterior shots of Grimmauld Place in Harry Potter and the Order of the Phoenix (2007) and Deathly Hallows Part 1 (2010). It is also an old resevoir, which was developed between 1821 and 1828 and is currently supplied by the Thames Water Ring Main.
Number 12 Grimmauld Place is the home of Harry’s godfather Sirius Black and is concealed by magic between number 11 and 13 – invisible to Muggles.
After the return of Voldemort at the end of Goblet of Fire, the house becomes the headquarters for the Order of the Phoenix and is later a hideout for Harry, Ron and Hermione in Deathly Hallows Part 1.
Find Claremont Square on Google Maps here: