A 19th century flat on one of London's prettiest squares

The interior designer Virginia Howard was not intending to move from Knightsbridge to Pimlico; you might just as well have suggested relocating to Acapulco. But when a friend told her about a first-floor balcony flat in one of London's prettiest garden squares, laid out by Thomas Cubitt in the 1830s, she was persuaded to take a look. 'In five minutes, I'd made up my mind to buy it. It was one of those life-changing moments.'

Although the flat had been the piano nobile of an elegant, Grade II-listed stucco building, not everyone would have seen its potential. 'It was dire,' says Virginia. 'A ghastly Sixties development. They'd crammed in three bedrooms, and lowered the ceilings to save on heating. I wanted to bring back all the classical details, so there was a lot to be done.'

In the end, the work was completed over a two-year period. Her decisions about space were bold. The hall, for example, was enlarged to give an instant impression of airiness, whereas other designers might have made more of the kitchen that leads off it - now a narrow, streamlined galley. Two meagre bedrooms were knocked into one generous one, filled with light from the floor-to-ceiling windows, while the third was added to the drawing room as a dining area. This left no room for the home office she occasionally needs, so she devised a wall cupboard that opens to reveal a workstation. The ceilings have been raised to their original height throughout.

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Detailing has always been important to Virginia: 'It ups the whole game - particularly when you're working in a small space, because everyone can see it more easily; I spend hours and hours on it.' So, as you enter the flat, you notice the beading on the front door, the cornice she has added to the hall and the double doors into the drawing room. Sir John Soane has been a strong influence, as have two twentieth-century designers, John Fowler and David Hicks.

'What I really love is bringing old buildings to life for the twenty-first century,' says Virginia, whose previous interior-design commissions have included the eighteenth-century Southill Park in Bedfordshire. 'I feel most comfortable working with classical houses, but one has to move with the times and add modern elements.'

With her own flat, she has created a traditional backdrop but enhanced it with the unexpected. Of the drawing room, she says, 'I started with a neutral palette and then added the warm elements: a suzani tablecloth from Uzbekistan; ikat cushions; and red Claremont linen on the nineteenth-century French chairs.' A mirror above the chimneypiece adapted from a Henry Holland design at Southill works remarkably well with a Nigel Carew-Jones acrylic table and a set of Picasso lithographs - 'a birthday present to myself: I wanted to move away from the big oil paintings you might expect'.

Similarly, the hall, with its walls painted in Paint and Paper Library's 'Thames Mud', is enlivened by an ikat textile framed in Perspex. And in the bedroom, the pale green and yellow 'Tree of Life' curtains and headboard - made in her own colourway by Bernard Thorp - are offset by paintings of the Silk Route by Harry Holcroft.

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So does she have any regrets about leaving Knightsbridge? It seems not. 'In a square like this, you can believe you're in the country. I've come to love Pimlico, because it's such an easy-going place to live: it's not as stuffy as some of the so-called smarter areas. There's a bit of everything going on here'.

Virginia Howard: virginiahoward.co.uk