KEEPING FIT fit has gone beyond the fad stage. Basements are being converted not into recreation rooms but into workout rooms. Where once the family room was pine-paneled and given a wet bar, it's now more than likely the site for rowing machines, indoor jogging tracks and Nautilus equipment.

One Silver Spring professional couple (she's an attorney, he's a physician), have a particular exercise passion: swimming. To keep fit without losing time, the two decided to build an indoor pool -- not a big showcase pool, but a serious lap pool. The one-story addition that houses the pool runs the width of their home. The pool itself is 10 feet wide and 35 feet long and varies from 3 to 5 feet in depth.

Although swimming means business in this house, the couple also wanted it to mean pleasure on other occasions. They wanted the pool area to be homelike, not chlorine laden, spartan or reminiscent of YMCAs. Designer Marvin Liberman chose for them attractive soft lighting and hanging textiles that won't get damaged by moisture in the air. And while the pool's primary function is for a good daily workout, it is also a wonderful place for entertaining.