This festival is usually over a single weekend but this year, to celebrate 30 years, it is running from 8 to 21 September.
The event list goes live on 24 August but you can get an idea from the 2021 list here
https://openhouselondon.open-city.org.uk/listings.
Also see
https://open-city.org.uk/open-house-festivalAll these events are free and you can get to see loads of stuff that is not usually open to the public. Last year 10 Downing Street was opened up (free but by ballot due to space) and some stuff needs pre-booking.
In 2021 there were 11 events taking place in Greenwich alone, including tours of Charlton House for example.
