Open House London (September only)

An annual city-wide architecture/design event started in 1992 by Victoria Thornton, Open House London took its inspiration from the French, who since 1984 had been giving the public a once-a-year opportunity to visit private and public buildings, monuments, private homes and sites, many of which are normally not accessible. Normally takes place on the third weekend in September — with many buildings open either Saturday or Sunday — so plan ahead. Certain high-security, ultra-popular and/or exclusive sites require prior booking, and it's best to do this in early August as spaces go annoyingly fast. Booklets can be found free at local libraries.