Open House London is one of London's most popular events and gives members of the public a chance to look inside some of the capital's finest structures, old and new.
It takes place annually, over one weekend (17th and 18th September), and first started back in 1992. Since then it has grown in popularity, and has allowed Londoners and visitors alike to gain an appreciation of some of the extraordinary architecture the city has to offer.
Many (though not all) of the Open House events require you to book in advance, and for some places you'll have to be quick off the mark. Many of the buildings involved in Open House London aren't ordinarily open to the public, so it's not difficult to see why many tours quickly become fully booked.
This year more than 700 buildings and locations across the capital are opening their doors, including architects' homes, eco and retrofit buildings, regeneration projects, landscape projects, government buildings, historic landmarks, major infrastructure sites and more. And each and every one of the events is free.
Some famous places involved in Open House London this year include Tower Bridge, Kew Gardens, Kensington Palace, BBC Bush House and the Lloyd's of London building.
But it'd be a shame to simply stick with the high profile locations, for some of the best Open House offerings come in the form of lesser known places - the hidden gems of London, if you like.
Open House London's comprehensive website allows you to search through the listings by name or post code, and there's even a free iPhone app available, enabling you to do some planning while you're on the train or bus. It's really recommended you spend some time looking through what's available - the research will be worth it.
And don't forget the London Night Hike, in aid of Maggie's cancer centres. Last year the event saw around 2,000 people taking part, walking a 10- or 20-mile hiking route (it's your choice) through London, past some of its best architecture. The event raised £400,000 for Maggie's cancer centres. Click here to find out how you can get involved in the Open House's London Night Hike.
[Top image: Ian Mansfield]