Now that the kitchen's done, why not remodel the bath?

The bathroom, once the home's most standarized, taken-for-granted utility, is in the midst of an improvement revolution that is recasting its size, look, and range of functions, and making it the focal point of ingenious decoration.

The National Kitchen and Bath Association is estimating that, during 1983, almost twice as many bathrooms as kitchens will be remodeled and refurbished. People who have already done their kitchens are now tackling their bathrooms.

Dave Sauer, publisher of Qualified Remodeler magazine in Chicago, and the National Association of the Remodeling Industry both estimate that $2.4 billion will be spent on bathroom projects this year. This figure represents an ''installed price,'' which includes materials and labor for cabinetry, plumbing fixtures, and so on, but does not include accessories and linens.

The Bed and Bath Association included linens and accessories in its research, and came up with a total estimate of $4 billion that will be spent on bathrooms this year.

''In the past year,'' says Susan Ames, a decorating consultant with AT&T, ''the bathroom has emerged as the most talked about and photographed new design and life style area of the American home or apartment. Some real estate agents even go so far as to say that a glamorous bathroom sometimes now surpasses the kitchen as a decisionmaking element in the purchase of a new home.''

Recently, Villeroy & Boch, a West Germany ceramic producer with United States headquarters at Pine Brook, N.J., convened an advisory panel of interior designers. According to this panel, if there is one paramount new trend shaping residential design it is the current attention being given to bathrooms by the American consumer.

Marjorie Bedell, a West Coast designer on the panel, said, ''Clients today are more vocal about the bathroom than about any room in the house. People will forgo a bedroom, but not the right bath.'' The panel noted the trend toward more spacious bathtubs, taller lavatories, roomier basins, new matte finishes on fixtures, and the trend away from white fixtures toward a new neutral shade, and soft pastels called jasmine, magnolia, and crocus.

It isn't all luxury, however. Dave Sauer of Qualified Remodeler underlines a reality when he says, ''While glamour bathrooms are getting most of the media attention, there is a prosaic reason for a lot of the remodeling that is going on. That reason involves the normal aging of the existing housing inventory. Of the 80 million existing houses and apartments in the country, about 60 percent are over 20 years old. That means many bathrooms have aged to the place that they are ready for replacements or refurbishing.''

Last month the Kohler Company invited six designers to come up with new bathroom decorating ideas and introduced glamorous baths ranging from French Country and Japanese to avant-garde modern and neoclassic.

Company vice-president Mike Kohler notes that bathrooms are now getting a lot of the amenities formerly reserved for other parts of the house. He cites the new appeal of light (not white) fixtures, and says Kohler's new neutral color, called ''parchment,'' has won strong approval. One setting showed the new, soft wild-rose pink; another, gleaming black fixtures.

Meanwhile, the recent exhibition of the National Association of Home Builders in Houston showed not only a vast variety of the more standard porcelain fixtures, but also a multitude of hot tubs, spas, Jacuzzis, and saunas. American-Standard introduced two new spas (a combination of hot tub and whirlpool), and vice-president Richard Mather explained, ''We know from our market research that there is an increasing desire for spas for the American home. Homeowners who in the past might have considered a swimming pool are now considering spas as a recreational alternative.'' The company's larger spa measures 86 inches wide by 39 inches deep.