London Open House Festival 16 + 17 September

Testee

The festival gives everyone the opportunity to visit over 600 buildings new and old across London – many of them normally closed to the public – completely free of charge.

http://www.londonopenhouse.org/london/home.html

Lots of great buildings to explore although some need to be booked in advance but there are still loads where you can just walk in and wander round.

You can pay £3 for a PDF containing details of participating buildings, but there's also a free online database, accessible by following the links to Online Buildings Search. Once you've brought up a particular borough's listings, click on the building's name to find out details of what's accessible, the opening times and, if necessary, the booking protocol.

DrFluffy

I love Open House. I haven't checked ou this years brochure yet, but have been to the last few. A great hidden gem is the medical school library of Barts and The London Med School in Whitechapel... It's a converted church and stunning.