“Oh!” I hear someone say. “It’s just an ordinary bathroom!” I detect a note of disappointment coming from upstairs. In my haste to appease the waiting crowds and get the queue moving, I have forgotten to rope off part of the upstairs – and have accidentally allowed visitors a glimpse of the squalor that is the family bathroom, complete with broken showerhead, unscrubbed bath and an array of squished toothpaste tubes and oozing bottles of cheap shampoo. Next door to that, also visible, is a child’s bedroom jam-packed with all the junk that we have removed from downstairs to achieve a lovely minimalist look. The illusion that we live in a perfect, clutter-free show home is shattered.

As I discreetly usher out the stray guests and close the door on our everyday mess, I ask myself: why would anyone allow hundreds of strangers to traipse through their private living space? Why would you turn your family home into a museum for a day and have hordes of sightseers examine your kitchen cupboards, bathroom, even your bed?

And yet people do. Open House, London’s festival of architecture, has been showcasing good design since 1992, allowing the public to see inside buildings not normally open to them. This weekend, 800 venues will take part, including government offices, company headquarters, theatres, cultural centres and schools. About 10% of the buildings are residential. “Originally, we knocked on people’s doors if their house looked interesting,” says Victoria Thornton, the project’s founder. “Now we do research. And sometimes we’re approached by architects and individuals who want to share their designs.”

So, while there will be opportunities to see inside some iconic landmarks, including the Bank of England, the Gherkin, 10 Downing Street and Lloyds, there will also be more humble buildings on show – like my modest south London home in Walters Way (see “Life on stilts”, overleaf) and these three others…

A wow! in every room

St Pancras Chambers:part of the Victorian gothic St Pancras station and hotel (NW1 2AR)

Peter Tompkins is not surprised by the constant interest in his home. He lives in the clock tower of St Pancras station – designed by Sir Gilbert Scott as part of the Midland Hotel, which opened in 1873. His dramatic three-bed flat, with its vast 10m high living area, is frequently rented out for parties, weddings, photo shoots and overnight stays. It’s easy to see the appeal: the space makes full use of the original beams and Scott’s stonework and brickwork, brought down from the Midlands by the Midland Railway company.

“The quality of craftsmanship is so good,” Tompkins says. “It’s always a pleasure to point out the fine stone carving and excellent brickwork. No one would build anything so preposterously expensive today.” Some people might be daunted by the prospect of furnishing such an architecturally stunning space, but Tompkins explains that he tries not to detract from the Victorian brickwork by overdoing it. “I try to have one thing in each room that makes people go ‘Wow!’, like a special headboard or some remarkable LED lighting.”

Living in a Grade I-listed building can, however, have its drawbacks. “The belfry is very cold in winter. The first thing I did was put in secondary glazing. But even so, it’s difficult to make it warm.”

Open House visits full but viewings can be arranged with the owner via stpancrasclocktower.co.uk

Utopia with en suite Shanks

Highpoint, Highgate: one of London’s finest examples of international modernism (N6 4BA)

“I love the utopian thing,” says Carolyn Parmeter of her home, which is the work of a famous duo: the visionary architect Berthold Lubetkin and the revered engineer Ove Arup. Built in 1935, Highpoint’s two apartment blocks were intended as a prototype for a better place for Londoners to live, away from the city smog. “It’s still an incredibly convenient and enjoyable way to live,” says Parmeter.

Designed to encourage collective living, the flats provide residents with shared gardens, tennis courts and a swimming pool. Idyllic as it sounds, there are drawbacks to living in such a historic building: “Our service expectations are different today,” says Parmeter. “The plumbing is a constant challenge and the kitchens are quite tiny.”

She has done Open House for 10 years and this year is opening for pre-booked guided tours. At first, she admits, opening up your home is quite terrifying. “I like to think it makes me get the flat in shape. It’s a good incentive to do all those little jobs you’ve been putting off for a year. Not that we don’t love our homes, but sometimes you need to be reminded of the wow factor.

“Over the years, I have got really relaxed about the public looking in private spaces. One of the special things about Highpoint is the bathroom design. Shanks made one especially for the block – it’s called the Highpoint suite.”

If fully booked, it may be possible to arrange viewings with the 20th Century Society

Mews with a disco attached

Russell Gardens Mews, Shepherd’s Bush:a sustainable mews house built in 2011 with an underground extension, boasting nightclub, cinema and spa (W14 8EU)

The first time Claire Farrow did Open House, in 2001, she was still living in a flat and her first visitors couldn’t get in. “We stayed up until four in the morning getting ready and ended up finishing it off on the day. There was a massive stainless steel planter blocking the front door and we had to get the visitors at the front of the queue to help us move it before we could let anyone in.”

Farrow and her husband, Ian Hogarth, now show their new house, which they designed themselves and moved in to five years ago, the build having featured on Channel 4’s Grand Designs. The end-of-terrace near Shepherd’s Bush includes a nightclub, cinema, spa and sauna all hidden beneath the back garden. The family are used to strangers entering their home, since they use it to showcase their architecture practice. “There were 1,500 visitors for last year’s Open House and we always have queues all the way down the mews,” Farrow says. “Everybody is incredibly kind and sweet. The one strange thing was when I found a young man in my wardrobe, filming inside.”

Life on stilts

Alice Grahame on her house in Walters Way, Lewisham: a close of 1980s self-builds (SE23 3LH)

Walters Way is a cul-de-sac of 13 timber-framed, flat-roofed houses built on stilts, completed in 1987 as part of Lewisham council’s self-build scheme. The architect Walter Segal led the project and developed the post-and-beam method that empowered locals on the council waiting list to build their own houses. The stilts mean they are closer to trees than conventional houses.

The close has been taking part in Open House since 1989, when the houses were just two years old. Initially, two of the self-builder families opened their homes, keen to share the excitement of what they had achieved. Today, the tradition is continued by newer residents, who bought rather than built their homes. Some families who now live in the street were Open House visitors: they liked what they saw so much, they waited for a house to come on sale.

Every year, we welcome about 400 visitors. Although there are only two houses that people can go inside, it is really the whole street that is on show. Neighbours serve tea and cakes, sell plants, and chat to visitors who comment on how rare it is to find such community spirit in London.

Some visitors are ardent Segal fans. While self-build has never gone mainstream, there is growing interest given the housing crisis. Some want to see how the homes have been upgraded to make them more eco-friendly. Others are locals who have seen our houses from the main road, down an unusual turning marked as a private road, but have never been in.

Many Open House visitors say they are inspired by what they see but last year one guest was spotted crying, saying she lived in a tiny rented bedsit and couldn’t afford to eco-refurbish her home. It was a reminder that good design and home improvement are only really available to those lucky enough to be on the housing ladder.

• Open House London is on 20-21 September. Details: londonopenhouse.org