IT'S A SPA, IT'S A GARDEN, IT'S `SUPERBATH'. Today's bathrooms are styled far beyond yesteryears'

Many home buyers are demanding more and more glamorous bathrooms. And builders and home remodelers love supplying them. They answer the needs - as well as the fanciful expectations - of people who are no longer satisfied with practical, plain-Jane baths.

Today, many bathrooms serve as personal havens, relaxation retreats, or fitness centers.

Some are decorated with art, antiques, and Oriental carpets. Others are veritable solariums full of foliage and flowering plants, huge hand-thrown pots, and bibelots - all reflected in big gilt-framed mirrors.

Many modern baths feature such amenities as superb architectural detailing, decorative columns, large windows and outdoor views, skylights, and cathedral ceilings.

Top architects and interior designers have applied their ingenious thinking to bathrooms, and leading magazines and decorator show houses that heralded their efforts.

For many, the ``superbath,'' with all its embellishments, has become a vital element of today's design-conscious life style.

It is estimated that between 5.2 and 6.5 million ``bathroom remodeling and fixture replacement jobs'' will be completed in 1988.

Ed Pell of Kitchen and Bath Business magazine says that between $9 and $11.6 billion will be spent by homeowners this year to rework, repaint, and beautify their bathrooms.

Some people will spend as little as $300. The average remodeler may spend between $2,000 and $5,000 per bathroom.

About 2 percent will invest more than $11,000 to get the bathroom of their dreams, including fancy faucets, whirlpool tubs, his and hers pedestal basins, and floors and walls paved with stone like marble or granite.

While attractive new product designs emerge each year, it is a whole rainbow of colors that enliven the bathroom scene this season.

Although one company spokesman said, ``White is forever, my dear, and many top interior designers prefer it,'' the fact is that color has burst out everywhere - from creamy pastels, deep blue-green, gray, and rose - to black, silver, yellow, brown, and beige. Almost a rainbow of colors is represented in these selections.

John Laughton, manager of marketing projects at American Standard, says the industry continues to bring out new colors because of consumer demand as well as the influence of modern architecture and the fashion industry.

Builders, he finds, usually like to play safe on color, choosing off-white or one of the ``whisper'' pastel neutrals. White, he says, tends to be strong at the very top and the very bottom of the market.

While most people who are remodeling bathrooms spend from $500 up to $3,000 for a new tub, lavatory, and toilet, Mr. Laughton calls whirlpool tubs ``really big business today.''

The top-of-the-line deluxe whirlpool tub made by American Standard retails for $25,000, and it includes a complete home entertainment and communications center as well as a home security system.