“A London address is, perhaps now more than ever, a central requirement for many of the world's super-rich in terms of their global property portfolios”. So begins a report from the upmarket estate agent Knight Frank last week. We're talking about “high net worth individuals” (HNWIs): those with net financial assets of more than $1million (£503,000), excluding their primary residence. By 2006, the ranks of HNWIs in the UK had swelled to 485,000, and that year alone they increased the proportion of their portfolios dedicated to property from 16 to 24 per cent, according to Merrill Lynch.

But what sort of property? Grand London townhouses rarely come up for sale, so wealthy househunters with an entourage (particularly foreign buyers, who account for 60 per cent of all Central London property purchases above £4 million) are increasingly seeking flats with separate staff quarters, such as Flat 4 at The Manor on Davies Street in Mayfair. Here, your valet can retire below stairs, Gosford Park-style, to a dark room next to the coal hole in this imposing Edwardian apartment block round the corner from Claridge's.

You enter the large, four-bedroom flat, for sale for £5million through W.A.Ellis (020-7306 1610), via an ornate lobby, complete with porter and roaring gas fire. The flat has an unusual layout, with lots of small, quirky rooms that link into each other, and a grand octagonal dining room. You might like the florid Baroque decor, or you might not. But the buyer (who the agent, Dan Wiggin, “can't see being British”) will doubtless be pleased to note that it is all on one level (“lateral is very desirable”) and that there is a separate staff entrance in the back kitchen. The lease is short, only 53 years, but Wiggin assures me that the price reflects this.

If your maid is diminutive and you think £5million a bit steep, W.A.Ellis has another flat with a tiny separate bedsit at 30 Eaton Place in Belgravia for £3.25 million. It's run-down, hence the price, but has a huge drawing room that runs the full depth of the building, three small bedrooms, a private terrace and a 115-year lease.

Staff accommodation is being included in high-end new-build flats for the first time since the 1930s, according to Wiggin. Then, smart Poirot-esque mansion blocks “often had a room behind the kitchen for the maid, but this was dropped after the war as no one could afford staff”. Now, he says, “there's a lot more money sloshing around and luxury developers are building staff quarters again, even in flats.” Where this is not possible, as at One Hyde Park and St Pancras Chambers, developers cut a deal with a nearby hotel for five-star room service.

It's not just staff quarters that interest millionaires. According to Knight Frank, “it is hard to claim exclusivity in an area where you are surrounded by wealth. A new market has been created as buyers find needs being provided for that they never thought they had.” So even without a fleet of staff you can still keep ahead of the Joneses. A two-bedroom, two-storey flat down the road at 19 Eaton Place, for sale for £5.5 million via Savills (020-7730 0822), comes complete with luxury bespoke interior decor and a “wow-factor” roof terrace. The designer Kamini Ezralow (020-7349 8020) has created a masculine Art Deco look using leather, chrome, antiqued glass and animal-print wallpaper. It will all cost you at least £250,000 extra but, as Charles Lloyd of Savills explains, “people at this level are too busy to browse round Habitat”.

The terrace is the deal-maker, though, because hardly anyone else on the street has one. “It's very rare for the Grosvenor Estate to give consent for a roof terrace,” says Lloyd. “It adds dramatically to the appeal and the value.”

Why does any of this matter to the rest of us, you might ask? Because we'll all be copying these trends soon, according to Knight Frank: “the trends established by the super-prime market serve ultimately to influence the prime and main markets over time.” So, get ahead of the curve and recreate the look of the Eaton Place roof terrace in your own back garden: the lanterns cost £50 and come from John Lewis.

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