A London flat by Adam Bray full of rich colour and texture

The American businessman was pretty pleased. He loves visiting London and that very day he had completed on the purchase of a one-bedroom flat on the top floor of a house in central London. But his satisfaction was short-lived. As he opened the front door of his new flat, an estate agent emerged from the one next door, which was also for sale. In a trice he knew that one bedroom was not enough. He had to buy the other flat, too. The purchase was the easy bit. The negotiations for combining the two flats took a tortuous two years of discussions with the Grosvenor Estate, which owns the square the property sits on.

Luckily, he had a strong team on his side: the interior designer Adam Bray and the architect Eugene Hanniffy of Garnham Wright Associates. With the building permissions finally approved, their task was to turn the warren of small rooms, which would have been servants' bedrooms when the house was inhabited by one family, into an agreeable three-bedroom home from home.

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'The owner had very strong views on what he did not want,' says Adam. 'In particular, no white and no leather. So we approached the first meeting to show him the fabric choices and the design for the new layout with a certain trepidation.' But the team had got it right. 'Hardly anything changed from that initial concept meeting to the final installation,' he says. The designers Nick and Joanna Plant, who frequently collaborate with Adam, worked with him on the schemes for this flat, too.

Different shades of a greenish blue on the walls provide a unifying theme throughout the flat. In the open-plan sitting room, it appears in a silk wallpaper by de Gournay, which makes a foil for the raspberry Le Manach 'Toile de Tours' upholstery of the sofa and ottoman. Placed around the tiny dining table nearby, a pair of Twenties French ebonised and gilded chairs, in their original orange silk velvet upholstery, also sing out against this background. The plain oak floor and the rough texture of the Beni Ourain Moroccan rug provide contrasts in texture to the silk and gilding. 'Keep the background basics simple,' says Adam.

One of a pair of eighteenth-century Chinese 'moon-gazing' stools - traditionally glowing coal would be put under to keep the seat warm while contemplating the moon - stands beside the sofa, its vivid blue picked up on the opposite wall in the surround of a Fifties Italian mirror, which is above a marble chimneypiece designed for this space. The armchairs are covered in a woollen fabric.

The main corridor leads from the sitting room to the dressing room, where the two facing walls of fitted cupboards continue the blue-green theme, this time in a gloss finish. 'I love gloss walls - they really do bounce the light about,' says Adam. 'But it is rare to get a client who will agree to them.' The carpet in here, designed to look like antelope hide, is from Capitol Carpets of Chelsea. The Italian walnut chest of drawers, tortoiseshell clock and mirror are all from the owner's collection.

Tortoiseshell also makes an appearance in the sumptuous guest shower and cloakroom at the end of the corridor. A 'scarlet' tortoiseshell triptych mirror hangs on sultry aubergine-lacquered walls. 'We did it the proper way, with 10 coats,' says Adam. The richness and depth of this finish, the beautiful lighting, and the floors and trim in Calacatta marble, make this tiny space a dream of luxury. There is more of the same Calacatta marble in the main bathroom next door, combined with simple panelled walls, a silver-framed wall of mirror above the bath, and a pair of basin mirrors framed in dark green glass, designed by the team, but inspired by a vintage Italian original.

Blue-green gloss reappears in the main bedroom, which adjoins this space, though this time in a muddier version. Here, the striped silk curtains and headboard look matt in contrast, as does thick jute carpet in a gutsy herringbone. A pop of vivid green from a lacquer bedside table and an elegant plaster lamp with vellum shade are all reflected in a Maison Jansen-style mirror. Eugene has made a virtue of the strange voids and beams of the ceiling, with an elegant coffered solution which, as Adam puts it, 'gives the room a bit of topspin'.

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The tiny, mirrored television room, which is across the hall from the sitting room, is much more vivid. A deep banquette sofa in Adam's own purple 'Greville' mohair velvet is ideal for lounging on while watching. There is however no lounging in the kitchen, where a baby La Cornue cooker, with a copper and steel trim, means business. A giant crane-like travelator conveyed all the furniture up to the flat in just half a day - from the classical columns and stucco of the pavement to this beautiful eyrie, which looks out over the treetops of the square's lovely gardens.

Adam Bray is a member of The List by House & Garden, our essential directory of design professionals. See his profile here.