It's Open House again - the two days of the year when buildings in the capital welcome a curious public. Robert Nurden reports

From the slim, sleek lines of a contemporary hi-tech office to the decorative fantasy of a Victorian gothic palace, the choice of buildings to visit in London's annual Open House weekend is extensive. This year's event takes place next weekend, with every prospect that last year's record of 350,000 visits will be broken.

Some 540 buildings - private homes, historic houses, arts spaces, contemporary offices and studios, institutions, City banks and government buildings - open their doors in what has become a fixture of the London calendar. "The event provides a unique opportunity for fans of the built environment to snoop behind the walls of places they usually can't get into," says Nicolette Spera, of Open House, an architecture education charity.

Open House has come up with themes - regeneration, eco buildings, bird's eye view, learning spaces and open sites - to guide you through. For your two days of legitimate nosey-parkering, you'll need three things: a weekend Travelcard, a good pair of shoes, and Open House London's 64-page Buildings Guide.

Among the first-timers is Norman Foster's 30 St Mary Axe, better known as the Gherkin, for which the queues are expected to stretch round the block. Other new boys are four in-progress sites: Wembley Stadium, complete with its gigantic arch designed to light up when there's a football match on; St Pancras International; Terminal 5 at Heathrow; and in Southwark one of New Labour's 53 city academies. The curved eccentricities of the Empress State building are worth a look. A landmark on the west London skyline for more than 40 years, it was once the tallest building in the capital. Recently refurbished, it now boasts Orbit, a revolving 30th floor designed by Wilkinson Eyre.

Children can join in the fun at City Hall, Ken Livingstone's glass and steel HQ (another Norman Foster), which looks as if it's about to slip into the Thames. From 10.30am until 1pm on Saturday, kids over six can design a town in an event called City of a Thousand Architects. But you might want to concentrate on one area - all but three of London's 33 boroughs are taking part. Tucked away in side streets are some gems, where home-owners have converted older buildings into gleaming modern designs. Ceramicist Lubna Chowdhary got top architect David Adjaye to plonk a studio in her back garden. In Kilburn, there's a photographic studio in Kilburn High Road with living-space above and eye-catching walnut veneer panelling. At the Learning Curve, Trouville Road, Clapham, Peter Romaniuk's remarkable curved glass family house is wrapped around a horse chestnut tree. "One of the great things about Open House London is making unexpected discoveries," says Spera.

Some buildings normally open to the public are offering something extra, such as an expert on site to explain the finer design points. At every venue, a volunteer will be on hand to guide, advise on disabled access, and answer questions.

Under the Meet the Makers scheme (buildings marked A and E in the guide), you can meet the architects or engineers, who will give talks or tours. And the Docklands Light Railway is running live, on-board commentaries about the buildings visible from the train.

We have Robin Cook to thank for allowing the hallowed halls and grand staircases of the Foreign Office to be opened up. Another oldie that's on my list is the 1930s former Daily Express building in Fleet Street, the best British example of the Art Deco style. Bart's Hospital Great Hall is probably worth a look, too. Alas, the one I really wanted to see - a straw bale home in north London - won't be on show: the owners are on holiday.

"We don't distinguish between old and new," says Spera. "Lloyd's of London is still considered contemporary even though it was built in 1986. This year there are more than 200 projects by contemporary architectural practices."

The 64-page Buildings Guide for 2004 is available, but order by Wednesday to avoid disappointment. Buy by credit card over the internet, price £3.75 including first-class p&p. Or send a cheque for £2.50 (payable to London Open House) and a self-addressed A5 (or larger) envelope with 42p in stamps on it to Open House, PO Box 25361, London NW5 1GY. Free copies are available from most London libraries. Further details on or by calling 0900 160 0061 (60p per minute).