Open House London: Who lives in a house like this?

For the past 18 years, every September, England's capital city has opened up its more tightly locked doors to the public for the event called Open House London. What started with 25 private buildings making themselves available for one weekend only to ordinary folk has now expanded to 750. Among them are 17th-century mansions, 21st-century schools, offices, architects' studios, even a couple of hospitals and hotels – plus the unbelievably sumptuous residence of the Argentine ambassador.

Every year, the Lloyd's of London building – the extraordinary office building for the shipping brokers designed by Richard Rogers in 1986, and still as exciting today with its external lifts and brightly coloured pipework – has queues round the block. This year, as a one-off, visitors will be welcome at the Commonwealth Institute in Kensington. It's the very last chance to see inside this impressive piece of 1960s architecture (English Heritage reckons it's second only to the Royal Festival Hall in importance) before minimalist John Pawson gets his hands on it and turns it into the new Design Museum, due to open in 2013.

But what we really want to see, of course, is inside those incredible architect-designed houses that are often stealthily slipped into tiny vacant plots on London's streets. Or the interiors of Victorian homes that may look conventional from without but have been spectacularly revised with glass and concrete within. This year, there are some exceptional examples, from a new-build in Borough to a 1970s duplex in north London that's been turned upside down.

Victoria Thornton, the event's director and founder, believes Open House to be a great way to make people sit up and pay attention to what good design contributes to our urban lives. And indeed, a tour around a newly built abode, or recently revised interior, is bound to be inspiring. But it's hard not to be as intrigued by someone's choice of soap, which tea they drink and the books on the shelves. After all, it is the details that make a real home and Open House lets you see them all.www.londonopenhouse.org; 17 - 18 September

Clapham House

This Victorian house in Battersea looked much like any other family home until architect Alex Haw got his hands on it a couple of years ago. What started as a fairly basic renovation job gradually increased in ambition as the clients, a sports writer and a teacher, began to want more and more changes. "At first, he was really progressive in his ideas, and she was resistant, but that didn't last," Haw says. "She is a talented amateur gardener, and has perhaps put the biggest imprint on the refurbished property, with planting that seems to grow right into the house from the garden."

What visitors should prepare themselves for is the show-stopping staircase, which is like a swirling piece of updated art nouveau. Digitally fabricated in sheets of MDF and oak, it arrived flat-packed, and was turned on site into this three-dimensional sculptural piece. "It's like a massive puff of smoke coming out of a genie's lantern," Haw says.

It's also worth checking out the skirting board – usually seen as a necessary evil by architects. It's been used as a device to show how the home's interior is a totally fluid and continuous one, as it tracks around the walls and finally flows out into the garden. Delightful.

www.atmosstudio.com

Frame House

"We house-sat here for two weeks recently," says Carl Turner, the architect who completed the Frame House in Borough earlier this year. "And thank goodness, it does work really well. Though we were so worried about the cat getting out that we kept all the big sliding glass doors shut, and having them open is half the point."

The Frame House is an ingenious use of a tiny site squashed between an old pub (now offices and flats) and a brand new housing-association block. "Since it was so narrow and vertical, we decided to make it like the tall, thin inner-city warehouses on four floors that used to be all around here." It consists of a simple concrete frame, strong enough for a further two floors to be added, should the client see fit and the planning permission come through.

The owners, who work in advertising and food photography, had some well-formed design opinions of their own. They'd rented a flat in the Barbican prior to the design of this house and were really influenced by the concrete finishes that surrounded them there. So here, you'll find exposed concrete ceilings and polished concrete floors. Even the stairs are in concrete, cast on site, and don't miss the recessed light fittings that were cast into the concrete slabs throughout. The house is full of the sort of fascinating details and finishes that show it to be a labour of love.

The pièce de résistance is the view from its roof terrace. Gaze out across the railway tracks, straight at the rapidly rising Shard and to the City beyond: London's grit and glamour perfectly juxtaposed.

www.ct-architects.co.uk

Kings Grove

"I remember one Open House weekend, going to see a little red-brick house on a triangular site in Brixton, just after I left college. It made quite an impression," recalls architect Joe Morris, who this year is opening the home he designed with his live/work partner, Mary Duggan, to the public for the first time. "The prospect of having people you don't know looking over your house is slightly daunting. But clients do it, so we felt we should, too."

Morris's house is a quiet but commanding building in carefully detailed brick, with windows so large, some walls are almost entirely glazed. But for all that it owes a debt to the European and American masters of Modernism, such as Mies van der Rohe, it sits right in the middle of two lines of classic English gardens, in Peckham, south London. "We are surrounded by sycamores, oaks and roses," Morris says. "It's like an urban jungle."

The house is as devoid of decoration as it could be. There is not so much as a door handle to disturb the continuous flow of space. "It's really just two rooms with no dividing walls," he says. When the couple first moved in, even furniture didn't spoil its light-filled calm. "We'd accumulated a lot of possessions by moving three or four times. But this time, we left them all behind, and arrived here with a couple of bags each and started again." They are acquiring the sort of mid-century modern furniture that suits the space. Cautiously, mind you. The beauty of Kings Grove lies in its purity, not its cushions.

www.dugganmorrisarchitects.com

Mansfield Road

The monumental social-housing development on Mansfield Road in north London, designed by strictly modernist Scottish architects Benson & Forsyth in the 1970s, isn't given much love by Camden Council, to whom it belongs. But it is adored by its occupants and none more so than Kevin Fellingham, a South African architect who has made his home here.

"The flats are fairly small, about 72 square metres," Kevin says, "but they feel so much bigger. When people first moved in in the 1970s, they brought their big old furniture with them and found it just didn't fit." Not that any of the original inhabitants would recognise their homes in Kevin's. He gutted the space entirely and started again, taking the living room up to the top floor, so it joined the roof terrace, which had been next to the child's bedroom. Now Kevin grows salad leaves, herbs and tomatoes up there in large buried pots.

Nearly all the internal walls are actually sliding partitions, which can be folded away. "I'll have to be here when people come round, to show them how everything works," Fellingham says. "In the summer, we open up the interior completely, and in the winter, we can seal off the bedroom, for example, so it's warmer and cosier."

The sleek, minimal interior – white plasterboard ceilings, painted concrete or rubber floors – would be spoilt by excessive decoration. But look out for a beautiful Senufo day bed from West Africa, carved from one piece of wood. "I bought it in Johannesburg 20 years ago when I was a student and I really couldn't afford it." He didn't know then just how perfectly it would fit into this north London home.

www.kfellingham.com