Basement flats: not ideal, are they? Dank, dark and redolent of the coal-hole, just the sort of property to lose value in the current market, right? Wrong. Two flats on the ground and “lower-ground” floors of period apartment blocks in SW1 have gone on sale: one, a three-bed in St James's, costs £9 million; the other, a four-bed in Knightsbridge, is on for (wait for it) £16 million. Well, £15.95 million to be precise - but what's fifty grand between oligarchs?

Both flats have two entrances: a communal one manned by a concierge which serves the whole block, and a private one, should Sir and Madam prefer privacy. Both have reception rooms on the ground floor, with the bedrooms below stairs, though the Knightsbridge maisonette (the agent spluttered at our use of the word “flat”) also has a master bedroom on the first floor.

This property, which is two neighbouring flats knocked together, is on Hans Place, a garden square off Sloane St, once home to Jane Austen and Shelley and near to the mothership of the international shopper - Harrods. It was bought last year by Richard Moore of Turnkey Estates, a top-end developer. Stretched over 4,200 sq ft, it boasts the lateral space prized by stair-averse foreign buyers. The interior has original panelling and bespoke handmade fittings and furniture. The best rooms are the drawing room and adjoining kitchen/dining room - huge and airy, with soaring ceilings.

Furniture is mostly grey velvet or dark macassar, including a pair of cabinets decorated with beautiful hand-embroidered silk panels and, inevitably, housing audiovisual equipment. A screen in the same silk divides the dining area from the kitchen and has another hidden telly: even tycoons, it seems, have solitary TV suppers. “It's very tacky to have too many TVs on display,” says Julia Selwood, the designer, who clearly made an exception for the master bedroom's en suite marble bathroom, which has flat-screens in the shower and above the sink. The bedrooms are lavishly upholstered, and the basement ones get a surprising amount of light. The walls are covered with £100,000-worth of art, the blinds are remote-controlled and there's a handcarved cocktail bar in the corner of the drawing room.

Worth £16 million? To a foreign buyer, says Richard Moore. “Many of the original owners of these houses were the international wealthy. It's a punchy price but these guys have this level of expectation. Anyone who turns this down at £15.95 million will wish they hadn't in 10 years' time.”

Noel de Keyser, of Savills, the agents, agrees that the price is achievable. “We sold flats in Eaton Square and Chesham Place last year for £15-20 million, and we already have serious interest from some Russians.”

Russians so dominate the target market for swanky conversions that developers are shifting the interior landscape of SW1 to please them. Our other basement, just off St James's Square, on King Street, is 3,500 sq ft of prime period property decked out with Swarovski light fittings and leather in all its forms. The developer, Manhattan Properties, is a 14-year-old partnership between Superna Sethi, who designs the conversions, and Harjit Singh, who does the interiors. They sell many of their projects before they reach the market, to existing clients who trust their work.

Almost all the furniture inside 1A King Street has been sourced by Sethi, mostly from Milan. Everything else was made in the Manhattan Properties factory in Italy, including the heavy oversized doors to every room. Brushed and buffed leather appears on the bannister, bedheads, door handles and inside the jewellery drawers in the walk-in wardrobes. (The one in the main suite would make Carrie Bradshaw swoon.)

Two of the three underground bedrooms have vast bathrooms with no windows and stone-lined walls. There are waterproof TVs at the foot of each large, egg-shaped bath, which sit within water features and are reached by stepping stones.

A £25,000 chinchilla blanket, softer than anything you've ever touched, will warm your toes in the master bedroom. Come the winter, basement flats might just be the ultimate in hibernation chic.

Contacts:

Hans Place: turnkeyestates.co.uk ,

King Street: manhattanproperties.co.uk . Both on sale via Savills , 020-7730 0822

FAST FACTS

The average property in the Royal Borough of Kensington and Chelsea (which also includes Notting Hill, Holland Park, South Kensington and Knightsbridge) cost £859,129 in April, according to the Land Registry.

A flat in K&C will set you back an average of £677,587.

Annual price growth has slowed to 16.3 per cent, down from a high of 31.3 in October 2007.

Only 127 properties were sold in the borough in February, less than half the figure for the same period last year.

31 per cent of K&C residents were born abroad, compared with the English average of 7 per cent, according to the council.

Times recommends

• Second homes: credit crunch casualty or money-spinner?
• How I built my own sliding house
• Top 10 incentives for house buyers