It's the annual opportunity to snoop behind the normally-closed doors of some of London's best architecture. Everything from the London Assembly building (known by some as Ken's Palace) to private houses (like the lovely 9 Wilton Way, Hackney, as featured on Grand Designs).
Here are a few examples from past LOH events:
[Above] Double Vision, taken from the roof of Plantation House on Fenchurch Street in 2004, looking towards Lloyd's and the Gherkin.[Above] Glass Ceiling of the British Museum, taken around the time of the 2003 event.
[Above] Making Her Point , members of the public on the big spiral ramp inside the London Assembly Building, during the 2005 event.
It seems I like to take mono images during these days out! Anyway, you can see some more in my Mono Selection 2 Gallery.