Gone are the days of frosted glass and functionality: in the most luxurious US homes today the bathroom is becoming more like a private health club than a hygienic necessity.

One of the most spectacular examples comes in a Manhattan triplex. The owners have converted most of what was previously an eight-room apartment into their bedroom complex. The lady's bathroom alone takes up 1,200 sq ft - three of the original rooms.

It is a place where cleanliness is next to costliness. The walls are lined with silk; the floors are slab marble. One tap alone costs $5,000. Like the main basin and tub, it came from Sherle Wagner, purveyor of bathroom fittings to the very rich.

The sanitary ware seems almost incidental given the rest of the equipment in the room. First, there is the television and the surround-sound stereo system. Like the curtains, they are operated by push-button controls. There is a sofa and love seat, plus a fridge and a microwave for the moments spent out of the water.

Dolly Lenz of Sotheby's International Realty, who is on the point of selling the apartment for more than $6m, says it is one of the two most luxurious bathrooms she has seen in Manhattan: "You could spend the whole day there without ever having to leave," she says.

The gentleman's bathroom is slightly smaller, but more lavish, with mahogany panels and 24-carat gold taps.

The shower room is large enough to take four people, each having their own, adjustable showerhead and a communal steam bar. The sellers, who own a brokerage house and electronics business, wanted their New York home to be as spacious as their other properties around the world. When Lenz was showing the apartment to the prospective new owners the bathrooms helped clinch the deal. "They said that was exactly what they would have done," she says.

Elsewhere in the US, it is generally second and third homes which are leading the way in bathroom luxury. "They are the places where people really go to relax," say Zackary Wright of Sotheby's. He is selling one house in Hawaii where the owners have a bathroom, which is partly indoors and partly outdoors. From the conventional shower, you go through to an area planted like a garden, with showerheads on the outside walls.

Wright says bathrooms are evolving as a result of the growing importance of the main suite in the most expensive US homes. Tubs are increasingly in the centre of the room, with space for more than one person. "The view from the bath is becoming increasingly important, especially if it looks across the ocean," Wright explains.

It was the view of the San Fernando Valley, which persuaded Karyn Miller to extend her house in the Los Angeles suburb of Northridge to allow for a vast master bedroom complex. It takes up about one third of the family house. Pride of place goes to the red Jacuzzi, set in front of a huge window, with mirrors to reflect the view. It has a tap like a waterfall to fill the 60-gallon tub. Music is piped through from the stereo system and there is a TV for alternative viewing. Next to the Jacuzzi is the shower and steam room, with glass walls looking through to the view, when the steam is turned off. There is also a sauna, as well as the more conventional basins and lavatories.

Miller has installed a microwave, so that the girl who comes to give her a massage can heat up the oil on the spot. "I like for her to come on a day when I can just stay in my room and not have to go out afterwards," she says. "I have a fireplace, which is lit by gas, but you can put on real logs as well. I light that, and burn lots of candles and incense. I just love it.

"I cannot travel anywhere else now without being disappointed," Miller goes on. "Even the Ritz Carlton cannot provide a bathroom like mine. When people come for the first time they are amazed - amazed and jealous." It is a far cry from the development of bathrooms in Europe, where his and hers washbasins are still considered a rare luxury. In London the most recent advance is to divide master bathrooms into three parts, with the lavatory and steam-shower rooms separate from the main tub.

At Culross Street in Mayfair, Wetherell is selling a newly converted house where the bathroom is also wired for sound - a tricky manoeuvre in the UK, given the strict laws governing electrical goods near water.

In Italy, home of the celebrated Roman baths, even these developments are far in the future. When asked for examples of luxurious bathrooms, the Rome office of Knight Frank said you should think yourself lucky if you got a chair.