Multi-millionaires don't decorate. Credit crunch or no credit crunch, when it comes to buying property, they still prefer to fork out for the luxury of fully fitted and furnished interiors than have to deal with builders.

The London trend for “turnkey” property has reached an unprecedented level of refinement. Niche developers catering to Belgravia tastes find that the more they spend on interior design, the greater the return. The popular look panders to playboy tastes: leather, dark wood and aggressive technology - the sort of place you'd expect to find Grand Theft Auto IV on the Playstation. Not every developer subscribes to this view, though the Candy brothers have made a fortune out of it, and it accords with the estate agent maxim that, while women decide mainstream property purchases, men have the final say when it comes to super-prime.

One developer, Finchatton, is firmly in the boys' toys camp. The directors, Alex Michelin and Andrew Dunn, are embarking on their most ambitious turnkey project yet. They have acquired the freehold of 63 Eaton Square, a Grade II-listed, 8,000 sq ft, seven-bedroom, six-storey mansion incorporating a mews house. Following a complete refit, they expect to sell in September for a whopping £30 million.

Freehold houses on the Grosvenor Estate (of which Eaton Square is a part) are very rare. Finchatton bought the lease two years ago when it was converted into flats, and acquired the freehold under new legislation which gives leaseholders of flats the right to enfranchise. “The Estate is now buying flats in each building to try to stop it, but they were too late with us,” says Michelin, who is spending £500 a foot to transform what was “a total shithole” into what will be “one of the best houses in London”.

Currently a building site, 63 Eaton Square will have grand reception rooms, a huge kitchen, a basement terrace accessed by stairs which descend at the touch of a button, a pool, sauna, gym, cinema, staff rooms, lift and an entire wall of temperature-controlled wine storage. A foreign buyer is anticipated.

If £30 million seems a bit eye-watering and you fancy something a bit more subtle, a new-ish high-end developer, Rigby & Rigby, yesterday launched 13 Cadogan Street, a 2,400 sq ft, four-bedroom, four-storey house with two private terraces, for £4.95 million (via Knight Frank, 020-7591 8600). Situated in a lovely Georgian row just behind the Kings Road, it is a new house behind a period façade.

Steve and Jenny Rigby, a husband-and-wife team, are ambitious. “We want to become the largest and best developer of turnkey houses in Belgravia, Knightsbridge and Chelsea,” says Steve. Despite revising the price down from over £5 million before putting it up for sale, he believes that this market will be comparatively well insulated from the economic slump.

The Rigbys have spent £460 a foot rebuilding everything from the roof to the foundations. A chunky dentil cornice runs round the ceilings, and bespoke cabinetry gleams from every corner. One curved walnut kitchen cupboard had eight layers of gloss applied to perfect its sheen. The technology (TVs, fridges, speakers), usually on boastful display, is also hidden behind wood cabinets.

It's not surprising to hear that the house is aimed at a couple rather than a bachelor. “Many of our competitors aim solely at young single men, but we are a lot broader than that” says Steve. “Jenny is very useful for the feminine perspective.” The occasional flash of colour is a welcome relief from the usual palette of neutrals. The drawers of the bespoke vanity unit in the master bedroom are lined with Tiffany-blue faux lizard skin. Is this the beginning of the end for beige?

finchatton.com; rigbyandrigby.com

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