Open House London is the annual festival in which the nosy public get to poke around in the capital's interesting buildings — many usually off limits.
As ever, there are hundreds (727 according to the app) of buildings and guided tours you might opt for, spread across most of London's boroughs.
The vast majority hold no barriers — just turn up on the day, queue if necessary, then take a look around the building.
Some of the more sensitive locations, however, do require prebooking. Some are allocated by ballot and others on a first-come-first-served basis.
Venues under ballot reportedly include 10 Downing Street, the BT Tower and (oddly) The London Eye — although at the time of writing, there is some contradiction between the app, website and booking form... best let this one settle down.
Those that require prebooking are numerous and include Tower 42, the Bank of England, Lambeth Palace, Antony Gormley's weird ROOM (a hotel room in a sculpture), Western Pumping Station in Chelsea, the underground bunker in Neasden, and the Government Art Collection, hidden in a warehouse off Tottenham Court Road.
The programme also includes the usual mix of talks, walks and boat tours exploring the capital's architecture.