Doubling space and value: London flat overhaul gave one couple a glass-sided courtyard garden in the heart of London

Doubling the size of your flat, totally reconfiguring it and adding a central, glass-sided courtyard garden open to the sky is about as good as it gets.

Deian Rhys and husband Euan MacLennan pulled off this coup with the skill and great ideas of architect David Money.

Now the couple live in a light 1,700sq ft flat with three bathrooms, two bedrooms and an elegant drawing room for the Steinway piano.

This duplex home is over the fourth and fifth floors in a stuccoed South Kensington terrace but it has a posh Parisian vibe, from its soaring rooftop views to the solid parquet floors, some original panelled doors, high ceilings and cornices, slick bespoke kitchen and double-width sliding doors.

Many more of these space-boosting transformations could work on top of London’s flats, mansion blocks and terraces to expand our housing stock.

You need determination to negotiate with the freeholder, which costs money, and can take years; to convince the planners, and in this case, to rehouse eight water tanks from the loft. But after all the hard work, the benefits speak volumes.

Deian, 38, a Welshman, is a solicitor with a fantastic eye for design and could see what could be achieved from their first viewing in 2008.

The couple met in 2002, as trainee solicitors. A few years later Euan, 39, a Scot, changed course and trained as a medical herbalist, now working for the NHS as well as being a director of the Pukka tea company. He loves plants and longed for outside space, ideally a country cottage garden.

In 2008 they married in Las Vegas. On honeymoon, Deian dealt with the paperwork for the South Kensington flat, which they’d decided to buy before jetting off, even though Euan says he thought: “Oh no, not another place with no outside space.”

The big top floor of an 1850s building had been unimaginatively converted. You walked straight into a narrow, sideways-on hall so the first sight was a wall. But there were high ceilings and bright light from two sides.

The clincher was that there was an excellent chance of extending upwards, exchanging the loft for a whole new mansard floor.

There had been a previous successful planning application, and there were examples along the terrace.

After talking to Kensington planners to see what was possible, they went ahead. They came home married and owning the flat.

Deian sketched out a top floor with a central courtyard to give Euan the outside space he craved. Passionate cook Deian had strong ideas on a glamorous, generous kitchen, finished in ochre and marble.

And on a large drawing room for the piano, a treasured legacy from his grandmother. Meanwhile, Euan wanted natural wood and stone everywhere possible.

They lived four years there before finding an architect through a friend. “We interviewed three,” says Deian. “David [Money] said he loved my design — but what about doing this and this, too?”

David’s brilliant ideas included opening up a dressing room behind the oak four-poster in the master bedroom; putting in skylights and creating dramatic wide, high, pocket doors.

He designed a fabulous timber screen up the stairs, with the theme continued in the master bedroom.

Working collaboratively has created a flat full of natural materials and tremendous style. The parquet that runs throughout adds glamour, and is acoustically soft.

But it’s that unexpected upstairs courtyard that makes your jaw drop. With sliding glass doors to the bedrooms, both of which have black-finished bathrooms, this is seriously special.

Demolition began early in 2013. “Nothing was left except the walls, front door and fireplace,” Deian says.

This build was as major as they get. Euan wasn’t impressed when he saw their future home reduced to four freezing brick walls with window holes. But after a few months, the shape and layout of the new floor emerged.

The entire build took just eight months, which is amazing for such a complex, high-spec job.

Even so, in the first week, the discovery of unexpected flues cost £10,000, wiping out their contingency fund.

But that was forgotten when the wraps came off. Euan says he sits in their new courtyard, sometimes with a croissant, sometimes with a cocktail, and knows that until the day he gets his country garden, he’ll be very, very happy here.