ABOUT THE only way people without pockets full of cash can manage to have a custom-built house today is by rebuilding what might be called a "used house" in a recycling neigborhood - preferably with the help of an architect held captive in the basement.

In this case, it wasn't the first time that architect Rich Heizmann had acquired living quarters under other than the usual landlord/renter basis. While he attended Catholic University's architecture school, he built himself a hutch. He installed the prefabricated living unit (complete with mattress, hot plate and parking space for motorcycle) in the drafting room of the architecture school with the forbearance, if not the approval, of the professor in charge, who wasn't that unhappy to have a free night watchman.

There were a number of episodes with the security police. Still, Heizmann managed to live rent-free through most of the rest of his academic life. During his last bout with calculus, and for the three years or so since he received his architecture degree, he's lived in the Bright-Sagnier basement, as their resident architect.

Theirry and Barbara Bright-Sagnier have given him (and his wife and now the brand-new baby) the basement apartment rent-free, and a bit of wages, in return not only for his design but for his supervision of the remodeling, and his own strong hands at the dirty work.

It came about this way. The Bright-Sagniers, after paying $39,500 for an Adams-Morgan rowhouse, decided that as willing and strong as they were, the house was too far gone for them to go it alone. So they called around to the architecture schools in Washington and asked, "Do you have an architecture student who's willing to take on our house?" The schools put the query up on their bulletin boards, three persons applied, and Heizmann was the one who had the courage to tackle the job.

The Bright-Sagnier job is not an easy one. Though they were living in the house, it was not what everybody would really call livable. There was, for instance, that Thanksgiving when Barbara Bright-Sagnier cooked dinner for 15 amid the rubble of the kitchen remodeling and washed the dishes in the only available place: the ancient clawfooted bathtub.

The five-floor yellow-brick house was built about 1917, one of the heydays for rowhouses. The Bright-Sagniers bought it because he liked the outside so much. And they liked the urban atmosphere of the Adams-Morgan neighborhood. It reminded them of the years both had spent in Europe. Besides, it was a 20-minute walk to downtown. And the garden, the sloping roof and the rear of the house faced south for solar heat. So they really didn't examine the inside of the house as closely as they might have. And indeed, when they did buy it, they found the kitchen was locked firmly and had to be battered open.

In addition to these not too-unusual problems - many rowhouses in what are called "emerging neighborhoods" need a virtual new house put inside - the Bright-Sagniers had some specific needs. Both are tall - Barbara is 5 feet 9. Thierry is 6 feet 1. Teen-age daughter Elizabeth is already 5 feet 6. They wanted tall ceilings, high doors and kitchen counters, and sinks and washbasins set at arm's height.

Not only that, they all play musical instruments Loud With groups. And company. Late. Both adults are writers as well. (She is with the European Community the Common Market. He writes books on home repair for Time/Life books.) There are times when one is playing a musical instrument and the other is writing.

Heizmann first lived in odd corners all over the house, wherever it wasn't currently being demolished. After he married, and the tenant in the basement moved out, he occupied that floor. In his spare time, he's been constructing a 20-gauge train complete with rail there. The train is big enough to ride on but may be too big to get out the cellar door. (But that's another story).

"What we knew we needed," said Bright-Sagnier "was an environment that was made for ourselves."

He and Heizmann have done the carpentry, the finished plumbing and some of the electrical work. "I think I'll get an electrician's license," said Heizmann. "Architectural jobs come and go, so it would be useful to have a craft as well. This job has been like an apprenticeship for me. In architectural school, you get a semester or so of mechanical systems, but that's nothing like having to make it all fit and work once you're actually on the job." Bright-Sagnier points out that all the electrical writing in the house had to be extra heavy so they wouldn't get hums in their extensive music sound systems.

Elizabeth and Barbara Bright-Sagnier have done most of the painting as well as other jobs. They've all become expert debris-movers. The rough plumbing, the heavy basic wiring and the boring and time-consuming drywall installation have all been hired out.

So far, they've finished the kitchen - hooray - and the top-floor music room/study. The master bedroom and another bedroom are wall-papered and comfortable but awaiting the fancy cabinetwork that is the house's hallmark. One bathroom is stupendous with a skylight, a pulley system for raising and lowering hanging plants and mylar-and-nailheads walls.

The kitchen is so good it is being pictured in "1001 Decorating Ideas" magazine. It isn't, as you might expect, your standard off-the-rack kitchen. In the first place, all the counters are indeed at Bright-Sagnier level. Standard cabinets were raised on six-inch bases. One whole wall is pantry, using an ingenious component system where shelves are affixed to the door (and extra hinges installed) with more shelves inside which pull out accordian fashion.

One great surprise is the powder room, tucked in the end of the pantry wall where you'd never guess, reached through a sliding wall that works on the barn-door principle.

Glass and china storage is tucked in a narrow space at the door to where the dining room will be. There are other useful and cheap ideas. The butcherblock counter-top was bought at a hotel auction, early, before the prices started going up. Its base is plastic pipe, painted black, put together with standard polyvinyl chloride connectors. The hanging support for the pots and pans rack is also PVC pipe, as is the base for the breakfast table.

The benches at the table are church pews. They bought 500 pounds of stuff from a church, piled it all into the car, and then a tire went flat. Guess where the spare tire was? Right, under the 500 pounds.

The trusses are made from wood torn out of the house. (A remarkable fence was also made out of leftover lumber. As Bright-Sagnier puts it, "This house is so old the 2-by-4s are actually a full two inches by four inches.") The pantry wall is exterior-grade grooved plywood. The color scheme is blue and bitter orange.

All the new window frames and most of the detailing here and there is curved. Because, Heizmann explains, "curves are sensious as well as restful." Anyway, in the bath a marvelous shape rises at one end of the bathtub to hold shampoo, et al. In the upper-floor study, the staircase ends in what can only be called a rolltop top. The horizontal (more or less) door is made of tambours. The bookcases form an arc. The housing for the music system is arched. The sofa in the study is curved at the ends like an Egyptian boat, with a ridged foam cushion made by Barbara Bright-Sagnier and stuffed (it took a day) by all. The place where the bath is going to go has a curved wall. There is a circular cutout between the second and third floors, near where the television-viewing bleachers are under constrution. And greatest of all is a south-facing six-foot round window in what will be the guest bedroom.

The rest is rubble, but, at this stage, organized rubble. The dry wallers are zipping along, and, God and the Union First Bank willing, it will all be more or less finished in about six months, four years or so after they all moved in.

And if it all has seemed rather like the set for "You Can't Take It With You" with all sorts of people popping up their heads like jacks in the box, and unexplained noises off stage, musical interludes, pratfalls, major and minor disasters, etc., well, it's all certainly been good experience. And isn't that what writers and architects are supposed to need?