When 400 buildings open their doors to the public next weekend - free of charge - under the London Open House scheme, among them for the first time will be the Bank of England, the Foreign and Commonwealth Office and the Scottish Office.
The scheme started in 1992 with a handful of buildings that were open to the public for one day, which was increased to two days in 1995. Victoria Thornton, project director and also director of the RIBA Architecture Centre, had seen for herself how successful such projects were abroad and was enthusiastic about making it happen here: "The idea is to increase people's appreciation of architecture, and to make the people working in these extraordinary buildings proud of them, too."
Now more than 20 boroughs are taking part, and the success inspired the Civic Trust to take up the baton and organise Heritage Open Days, taking place outside London this weekend: properties on display range from the Oxford Union and Cheltenham Ladies College to Barclays Group Archives in Manchester.
The building causing most excitement in the year's Open House is the recently restored Foreign Office. The Foreign Office had been intending for some time to admit the public, but the refurbishment was not completed until January this year. The Fine Rooms, through which the public will be allowed to walk, are stupendous: the Durbar Court, a three-tiered courtyard with Indian overtones and roofed in Victorian glass and ironwork; the Grand Staircase with its marble columns and gloriously politically incorrect murals (Britannia Peacemaker, for example); and the Grand Locarno Reception Room, with its high-lit, soaring zodiac-decorated ceiling: "I always think it's like a secular cathedral," says Kate Crow, historian and, appropriately enough, open government officer for the Foreign Office. Incredibly, in 1963, all this was scheduled for demolition.
Meanwhile, behind the famous windowless facade of the Bank of England (the "curtain wall" designed by Sir John Soane) is another extraordinary slice of gracious living. Visitors will be greeted by the gatekeepers in their top hats and pink frock coats and guided in groups of 25 along the mosaic pavement, past the flower-filled courtyard to the Court Room and Governor's Office. Oil paintings, leather seats, parquet floors and a waiting room which is more like a drawing room give an intriguing insight into the world of high finance.
At the other end of the scale, the 1930s Finsbury Health Centre (designed by Lubetkin) and Rotherhithe Youth Hostel are opening their doors. There's a pumping station in Southwark, a Buddhist temple in Wimbledon and London's third oldest synagogue in the garden of an 18th-century Spitalfields house.
Most visited last year were Somerset House and the former Midland Grand Hotel, which is now St Pancras Chambers. Designed by Sir George Gilbert Scott, the Midland Grand Hotel, with its towering staircase and grand Ladies' Smoking Room, had 8,000 visitors. It has opted out this year, in favour of other openings organised in conjunction with Open House.
Also popular last year were the architects' offices - those of Richard Rogers and Norman Foster attracted up to 2,500 people each. The scheme has generated a great following. "We've had letters from people who have done as many as 13 buildings in a day," Victoria Thornton says.
Christopher Salaman, a City of London guide, devoted a weekend to Open House last year. "There's a great appeal in seeing buildings that are not normally accessible, like the Old Treasury in Whitehall - wonderful." He also recommends the Economist Building and the former Port of London Authority (now headquarters for Willis Corroon Group).
An innovation this year is aimed at schoolchildren. A Passport to Design can be used by children visiting nine particular buildings, with questions to answer and spaces for drawings. Among other places, the passport will take them to the Foreign Office.
What has Open House got its eye on next? "The Prime Minister's home," says Victoria Thornton. "Why not? They do it in Paris."
Open House information line 0891 600061; Heritage Open Days (this weekend): telephone 0891 800603.
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies