When land is costly, previously ignored little scraps of space suddenly take on a new value. So in London, where prices are soaring and every borough is under orders from mayor Ken Livingstone to pile in masses more housing, you find developers scrapping like ferrets over every backland plot. We are no strangers to tall, narrow houses in the capital. But just how narrow can a house be and still feel like a real house?

The answer is, very slender indeed. A new three-bedroom terraced house in Maida Vale by architects Boyarsky Murphy, built for Geoff MacCormack, a musician/producer, and his family, is just 9½ft wide at the front, while a tall, wafer-thin house in Clerkenwell, built in 2000 by architect Jo Hagan and now selling for £950,000, is not exactly portly at less than 11ft wide.

To put those dimensions in context, a typical small-to-medium Victorian terraced house measures about 17ft across the front: basically, the minimum needed to get a front doorway and hall alongside a useable front room. So how do these super-narrow homes work?

In the case of MacCormack’s self-effacing west London home, it’s because it is built on a wedge-shaped plot. It widens towards the back just enough for the various rooms of a family house to be dropped into place. Still, it wasn’t easy: enough space had to be left to allow views out from the windows of the overlooking house alongside. Surely the neighbours would have lodged strong objec-tions? Luckily, the neighbour at the time was one Geoff MacCormack. He’d had his eye on the little single-storey former stable beside his duplex flat for some time. “It was literally outside my kitchen window, and I found myself looking down at this empty space,” he says. “I had always wanted to build.”

He and wife Sarah, and daughters Iraina, 21, and Adriana, 17, considered half a dozen architects suggested by the Royal Institute of British Architects before settling on the firm headed by Nicholas Boyarsky and Nicola Murphy. Apart from their design, which ingeniously brings daylight into the very confined space, “I liked the fact that they were a husband-and-wife unit,” MacCormack says.

The house may be compact, and was built for about £400,000, which is cheap for a family home in a London conservation area, but the project was scarcely straightforward. The basement — where there had been existing wine vaults — was excavated and built in concrete, while the upper floors were done as a sequence of stacked steel-framed boxes.

Each floor has a different shape and layout. Heavy negotiation with planners was needed: another storey was planned, but that was ruled out. Space was squeezed further when the front had to be set back; but at least that meant the family could have the luxury of a place to park a car out front. Despite this development, and the plot backing on to the concrete car park of a block of flats to the rear, they squeezed in a private, if compact, garden terrace. And despite the building work, they were able to sell their other flat and rent while their new home was completed. “We were able to sell it to an understanding guy,” MacCormack says.

The whole place — apart from the relatively large basement living and dining area — is an object lesson in making every square inch of space count. As you climb the narrow stairs through the four levels and peer into the various bedrooms and bathrooms, you are aware that there is only just enough room to make everything work, but the clever use of daylight makes everything feel bigger than it is.

The whole facade, for instance, is of translucent privacy glass: light floods in, but nobody can see in. “You have to design very carefully, you can’t just plonk anything down on a site like this. This one was like a three-dimen-sional puzzle,” says Boyarsky. The narrow plot led he and Murphy to call it the Sliver House.

In contrast, Clerkenwell House, as it is known, is totally straightforward. Narrow it may be — it was built on the site of a derelict single-storey bakery — but it is relatively deep, and it just goes on and on, up and up, seven storeys in all. There are three bedrooms with bathrooms, two living rooms, a utility room, a study — all the usual domestic facilities. It’s just that no room can be more than about 10½ft square, which is fine.

There is enough space for a two-person lift and a narrow staircase towards the back, which means that on all but the entrance level, rooms take up the whole width. Again, daylight and views are important. The higher you go, the bigger the rooms feel. It’s an illusion: they aren’t. By the time you get to the top, you have views front and back, and there is just enough room for a rooftop garden eyrie.

Matt Gibberd, of The Modern House, a specialist estate agency, admits that Clerkenwell House is not a home for everyone — particularly young families, what with all those stairs. Moreover, it’s very much inner-city living, no matter how fashionable the area has become. “You’ve got to like ‘gritty urban’ to live here,” Gibberd remarks as we gaze across the roofscape of this former industrial district.

Conceived by Hagan as his own home, in the end it was built for others. It has changed hands once, and, now the London pied-à-terre of Arne Maynard, a Lincolnshire-based garden designer, is for sale again, but has been stuck on the market for about six months, so its price has been reduced to below the psychological barrier of the £1m mark.

Personally, I love it. It’s as urban as could be, but in its vertically stacked way, it manages to be very different from your run-of-the-mill loft. Being so close to the financial centre of the City, I can’t see it staying unsold for too long. It is cleverly arranged, despite its many levels. “You only ever tend to use two floors of it at a time,” Gibberd says.

Actually, building ingenious houses on awkward plots is nothing new, as Jeremy and Jill Lever, who still live in a sliver of a house on a Notting Hill crescent, can testify. In 1973, an awkward gap between two of the area’s typical townhouses came up: it had planning permission for a traditional house. Jeremy, an architect, and Jill, an architectural historian, bought the site for £8,000 and spent £36,000 — quite a lot for the time — building a tall, wedge-shaped house. Barely more than 12ft wide at the front, it widens to 17ft at the rear, and rises five floors above street level. The couple even crammed in a two-level basement flat as well, and planners insisted on a double garage, later converted to a lobby and storeroom. So as not to renumber the entire street, the Levers’ home became No 29 and a half.

“It wasn’t easy,” recalls Jeremy. “The walls of the houses on either side leaned in slightly. As the house went upwards, we lost a valuable 6in of space.”

We can expect more of this kind of thing, if London continues to be an inflamed property hot spot. As Boyarsky puts it: “Small bits of land like this are a condition of the city. There should be a lot more of this kind of building going on; it’s a lot better than making more suburbs.”

Will every little ramshackle outbuilding soon be sprouting a mini-skyscraper, like a 21st-cen-tury version of Italy’s medieval San Gimignano? If prices in the capital keep spiralling up, it could just happen.

- Boyarsky Murphy Architects, 020 7388 3572, www.boyarskymurphy.com; The Modern House, 08456 344 068, www.themodernhouse.net

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