WHEN it comes to using interior space effectively, the McKenna family has got it totally worked out. They sleep on beds so high up their walls, they are close to the ceilings and utilise the space below. They work on laptops, eat, read, sleep, listen to music and watch television, often all in the same room.

A lot happens in the McKenna family’s stylish 1,200sq ft, two-bedroom, one-bathroom north London home. Indeed, at one stage, five of them lived there, which included a male au pair who slept in the “cupboard” bedroom.

Not only do the handsome family of four (Nancy, Bill and their student sons, William and Mark) all have their beds raised on bespoke steel frames 7ft off the ground, but shelves, storage space and televisions are also fitted at that level.

“The flat does not appear small to us because it has 14ft ceilings, and using that upper wall area means we have effectively added 50 per cent more space,” explains Bill, professor and clinical director of cardiology at UCLH. His wife, Nancy, adds that their methods mean the home is easier to keep clean. She also has an 800sq ft studio at the bottom of their garden.

Bill and Nancy used to live in Montreal but left Canada in 1976. “We came for a year,” laughs Nancy, a photographer, film-maker and founder of SafeHands, a charity dedicated to reducing maternal mortality and safeguarding the health of newborns. But then they found themselves staying, and in 1983 discovered this apartment.

“We bought it on the recommendation of an artist friend,” reveals Nancy, who, like the rest of her family, is clad in sports gear (Nancy did a degree in PE and met Bill, then captain of the university football team, through sport). “Another artist had been living here. Our kitchen was his studio, our boiler cupboard was his kitchen, there was no central heating and the property cost £60,000. Everything had to be redone.” Through painting the flat entirely in white, fitting a lot of mirrors to reflect the light and laying a light beech floor that flows throughout, the apartment feels spacious and contemporary..

There are also huge windows everywhere and clever lighting. “Most people design and furnish the house and then think about the lighting afterwards,” asserts Nancy. “But, probably because of my photographer’s eye, I think that should come first.” She worked on the lighting design with Maty

Grunberg, an artist friend who also created the beautiful pictures in the apartment. Many of the fittings (“mostly Italian, very expensive and classical”) still look great after 20 years. “Dimmers are essential throughout a small space, as they allow different moods at different times.” We are sitting in their fabulous Alternative Plans kitchen. It is the focus of the flat, with its white-lacquer Hacker cupboards, dark-grey basalt-stone work surfaces, stainless-steel splashbacks and real African-style fire. “Sometimes we cook on it,” says Nancy.

Juxtaposed with the clean, modern lines are old pine chairs and table (today it is being used as William’s office) and African wooden sculptures ‹ all symbolising fertility and pregnancy, reflecting Nancy’s involvement with the charity for safe motherhood and childbirth.

As we wander around their home she tells me that the au pair lived in the broom cupboardcum-boiler room (now used for storage) with a bed up a metal ladder. “We used to have a toilet in there, too,” laughs Nancy. “We always chose au pairs from big families because they would be used to lack of privacy.” Next door is the marital bedroom, which doubles as an office and library. Designed by Maty and executed by their friend, Asaf Tolkovsky, the frame of the 5ft bed is fashioned from steel.

Nancy is 5ft 9in tall. “But we’ve always had small beds. When we go away, the boys take it in turns to sleep in it.” Underneath the bed are beautiful his-and-hers cherrywood desks designed by Tolkovsky. The wall of mirrored cupboards has a flat-screen TV on top. “The only thing we don’t have in here is a gym,” jokes Bill. NEXT door, the boys’ shared bedroom has firemen’s poles, down which they shoot from their beds, which are themselves surrounded by curtains.

“The idea is to pull the drapes like on berths on a train. You get used to no privacy and sharing a bathroom ‹ one minute you are in there alone, the next moment Dad’s jumping into the shower,” laughs Mark.

Then there’s the sitting room, which pays homage to seminal designer Eileen Gray with its sofa, daybed, occasional table and coffee table.

Paintings and Nancy’s powerful black-and-white photographs of women and children are propped against the walls. “As I get older I become more minimalist. I prefer empty walls,” she says.

A woman with prodigious energy and a refreshingly unconventional approach to life, Nancy has cast her magic over the flat. “We have never wanted to move. Instead, we have bought homes for holidays in rural Spain and France.” Artwork by Nancy McKenna and other artists is on sale at the Art for Life Auction (proceeds to SafeHands), 6pm to 10pm 6 March, Mary Ward House, 5-7 Tavistock Place,WC1. Tickets from ndm@safehands.org.