Primrose Hill in north London is one of those places where nothing much seems to change except that once-crumbling Victorian houses are now glowing with new paint and worth millions, and ordinary shops have long been replaced with posh patisseries and gift emporiums. In planning terms, you would consider it the most conservative place on earth. But it isn’t. Not always.

The first all-new house in the area for donkey’s years has just been finished — and it is a stunner. What you get is what looks at first glance like a hovering, white-framed glazed box. This is almost, but not quite, an illusion. It does have a ground floor, it does connect with the earth. But what it is actually doing is balancing slightly improbably in the air.

There is a huge open space underneath one corner of it, acting as a sheltered entrance and car pull-in. You’d expect to see a column propping everything up at the corner. There is no such column.

But although the house indulges in such sophisticated structural gymnastics (courtesy of the engineering company Techniker), it does not make a big fuss about it, any more than it boasts about its architectural tricks.

Yes, it has a stupendous wide stone staircase sweeping you up from the entrance to the living spaces on the first floor. Yes, that does mean that it is an upside-down house, with the two bedrooms and bathrooms tucked in at ground level.

Yes, it has quite enormously panoramic windows front and back. And yes, it has some impressive purpose-designed furniture, including a vast polished-marble dining table. Oh, and by the way, the glass roof to the living area slides open if you want it to.

But in its way, this is a reticent little house. Because all the clever stuff is channelled into making a light-filled, open-plan but nonetheless home-like London pad for — who do you think? A rock-star couple? Some young banker or celebrity chef? None of those. Actually, it’s for the architect’s mum and dad. And they are, sort of, retired.

The architects are Matthew Springett and Kirsteen Mackay, the clients are Rod and Ros Springett. The new house is at the end of the long garden of the big old house the Springetts used to live in. The parents — designers both, though Ros is an artist these days — had moved to Ireland when their children grew up and left home, but kept the old house on.

Then the big old rotten horse chestnut at the end of their garden next to the garage finally became dangerous and they were told to take it down. That left a gap onto the lane behind. The elder Springetts had been thinking about downsizing to a London flat. Then they thought: why not build ourselves a smaller house instead? After that, it was a case of fencing off the plot, selling the old house with its reduced but still adequate garden, and building the new place with some of the proceeds — about £700,000 of building and fit-out work.

Because the lane behind already had a number of little mews-type houses on it, this one just slotted into place — with the advantage that it happens to look right down a cross-street.

Which is all fine and dandy — and the local planners approved it straight away — but, er, was this the kind of architecture the Springetts had in mind? “We’ve lived in three houses in London, and they’ve all been lovely examples of architecture of their periods, which means we’ve lived with decaying window frames and sashes and all that, but the interest for us here was being able — for once — to exercise our idea of contemporary living in the city. It was very exciting,” says Rod.

Ros adds: “We’ve always longed to be able to reflect the age we live in, and to be given this opportunity — and to have a son able to do it — is fantastic.”

Rod reckons the thing worked harmoniously because the whole family is visually creative: “It might not have worked. If I’d been an accountant, who knows? We might have wanted a different sort of house.” As it was, they knew what Matthew and Kirsteen were trying to do.

By that stage, they were getting their practice under way and had worked under the exacting eye of Sir Nicholas Grimshaw, the architect of the Eden Project in Cornwall, who lives nearby and had employed them for alterations to his own house. They had carried out commercial work as well as homes, and were starting to get established. So Springett Mackay tackled the house as if it was a key commission for a new client.

They were ultra-businesslike, according to Matthew’s father, who witnessed them whipping the builders into line and rejecting anything substandard. But what about from the architect’s point of view? Mackay says the Springetts were good clients “ because they had a clear idea of what they wanted. They were quite tough but professional — they didn’t change their minds once they’d made a decision”.

This is a house for living and entertaining, a sequence of interlocking spaces. A few sliding opaque glass screens and changes in floor material make token divisions between kitchen/dining, living area and study on the main floor (the study has a bespoke desk for each parent, facing in opposite directions), but essentially it is all one big milling-around space.

Indeed, the Springetts did not even want a garden, given that Primrose Hill is close by, but yielded to the planners’ insistence that the house should have a small patch behind it, treated as an outdoor room.

Downstairs, beside the super-wide entrance hall, is the main bedroom and a guest bedroom, separated by two bath/shower rooms with an interconnecting door.

There’s also a utility room tucked away in the centre, and that’s it. All very straightforward, really: a large flat expressed as a separate house, designed for people who want to drop in whenever they feel like it without having to worry about maintenance.

For me, what makes it so impressive for a work by such young architects is its air of calm assurance and restraint. Everything is handled confidently; it is luxurious without being opulent, and the result is a very sophisticated new home.

Grimshaw, a shrewd judge, reckons that Springett Mackay is a name to watch in architecture. On the evidence of this, I can only agree.

Springett Mackay, 020 7692 5950, www.sm-arch.com

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