Open House London is an organisation which promotes appreciation of architecture by the general public. It organises tours, lectures, educational projects for children and so on, but it is best known for Open House Weekend, a two-day event which takes place on one weekend each September throughout London since 1992. The event forms a London version of the European Heritage Days (Journées Européennes du Patrimoine), a Europe-wide event that started as Journées Portes ouvertes des monuments historiques (Historic Monument Open Door Days) in France in 1983. During this weekend many buildings considered to be of architectural significance open their doors for free public tours. Heritage Open Days is a similar event covering the rest of England, and takes place the previous weekend.
The Open House event in London is usually held on the third weekend in September. The 2010 event which took place on 18 and 19 September featured over seven hundred buildings, neighbourhood walks, architects' talks, cycle tours, and more. Well known buildings not usually open to the public which were open on Open House weekend in 2005 included Marlborough House, Lancaster House, the Mansion House, the Foreign and Commonwealth Office, and Horse Guards. The event attracts tens of thousands of people. It is often possible to turn up and go straight into some establishments, although queues are an hour or two long at the most popular buildings, while in other cases admission is by advance booking of a guided tour only.
The Open House event in London is synchronised exactly with the Europe wide event. The corresponding weekend outside London in the UK is held one week earlier (in 2010 11-12 September) but on largely the same basis.