Renowned landscape architect Kim Wilkie's London flat

When internationally renowned landscape architect Kim Wilkie was looking for a London pied-à-terre, only one location would do. 'I've always loved St James's,' he says. 'It's the perfect part of London. It has everything: the Royal Academy and the parks, you can walk everywhere, and pretty much all my clients have an office or a flat within five minutes of here.'

On the second floor of a large Italianate block of gentlemen's chambers on Jermyn Street, Kim's flat offers all the advantages of living in the heart of London with few of the drawbacks. Though only a hop, skip and a jump from the tourist-bedevilled maelstrom of Piccadilly Circus, it's a relatively quiet street that still retains something of its Georgian scale and character. As we talk, the bell of St James's Church, Piccadilly, clearly strikes the hour.

The flat, which he bought in December 2012, is flooded with light, thanks to five large south-facing windows, left uncurtained apart from discreet white-linen blinds. 'Three things sold the place to me,' says Kim. 'The windows, the proportions of the rooms and the entrance stairs.' Those stairs are indeed so grand in scale that Kim's modest front door comes as something of a surprise.

The previous owners, an elderly couple, had lived in the flat for many years, and the place was authentically Eighties in feel, with extravagantly swagged curtains, low false ceilings and laminate floors. The vast pink sofas they left behind had to be chopped into bits to feed them out through the door. That it looks so different today is thanks to the architect William Smalley, who Kim met when they worked together on the much-discussed Chelsea Barracks scheme. You can sense that Kim - quietly spoken, with intensely blue eyes - is the kind of person who thrives on peace and calm, and William has carved a series of fittingly serene spaces out of what was once an awkward plan.

MAY WE SUGGEST: An Edwardian villa by the architect William Smalley, imbued with his brand of quiet beauty

The most immediately striking component of his white-on-white scheme is the entrance hall, with walls that slope imperceptibly into a barrel vault overhead. Kim smiles when he describes it as 'very William'. The idea, William explains, 'came to me when I stuck my head up above the false ceilings in the original flat and saw an arch'. Two sets of plain openings, uncluttered by architraves, lead off to the left into the study and the sitting room, and to the bathroom and galley kitchen on the right. Recessed and pocket doors add to the overall feeling of quiet simplicity. The study, with two of the south-facing windows, also connects directly with the sitting room, which has a door to the en suite bedroom, tucked away overlooking a quiet internal courtyard at the back.

'I wanted the interior to look massive but simple at the same time,' William explains. 'Almost as if it had been carved out of a single space. The flat is also divided by two substantial walls, and I wanted the whole design to share that feeling of solidity and weight.' The effect is achieved by matching the depth of the solid walls throughout - in the form of oak shelves in the study, for example - but also with sturdy detailing, keeping each surface as simple as possible.

It's great to work with another designer,' William says. 'It means we're thinking in similar ways, though, of course, Kim has huge areas of expertise that I don't have. He was an ideal client in that he was able to explain the feeling he wanted to create in the space but didn't dictate specifics, such as what had to go where. At the same time, when I suggested things, he would keep sending them back until they were completely right.'

Kim has furnished the flat in a way that perfectly complements William's quietly austere design, with a collection of pictures, ceramics and furniture that reveals a deep interest in craftsmanship. The first things that catch your eye in the sitting room are the powerfully sculptural ploughshares, dug up from the fields around Kim's Hampshire farm and arranged on backlit shelves below a 'frieze' of decorative cast-iron gratings. Arts and Crafts furniture, outsize wood engravings by Julian Meredith and contemporary ceramics testify to his love of the handmade, as does the beautifully dovetailed ladder in the study, made for Kim by the sculptor Simon Thomas.

During our tour of the flat, William opens the fridge to reveal his client's culinary approach: it contains little other than bottles of Champagne. 'But living here I've no need to cook,' Kim protests. 'There are so many great restaurants nearby, and I've got Fortnum & Mason for a corner shop'.

William Smalley: 020-7242 0028; williamsmalley.com