JAYETTA Zimmerman had made up her mind she was going to buy a house that day in 1973.

"My mother was arriving with the money, but I thought I'd just test my choice by taking a last look at the want ads -- I'd been reading them every day for the last three years. So what did I see but this ad for a cooperative apartment with a 20-foot-high living room, three bedrooms and three baths. The ad noted it was 'just right for a single.' I didn't know then that the phrase translated as meaning a rough neighborhood.

"I called up the real estate agent. He said the house could only be shown on weekends, but I told him I was going to put an option on a house that afternoon. So I went to see the apartment and decided I had to have it. Mother and I looked at the house I'd originally chosen and we came back and I took the apartment. I negotiated quite a while and got it for $28,500."

Almost anyone with an ounce of appreciation would flip when they walked into the living room: a 20-by-20-by-20-foot cube with a handsome Arts-and-Crafts-style mosiac fireplace. A tall, wide window looks north across the entry garden of the building. A stained glass window was commissioned for the opposite wall, since the view is only of another building. Throughout the room are objects from the owners travels.

Less than a year after Zimmerman (and her two tenants) had moved into the apartment, Daniel Hecker, also a student at Georgetown University, came to study. When he first walked into the living room he said, "If a man asked you to marry him, how would you know if he wanted you or your place?"

Zimmerman replied: "I wouldn't put his name on the deed for seven years.

After eight years of marriage and two children, they're now co-owners.

The cooperative Meridian Hill Studio Apartment building, a sort of stucco pseudo-Tudor, was built in 1920 at the high point of 15th Street, by George Oakley Totten, one of the most famous of that era's architects. Totten was the architect Mary Henderson chose to design her huge Beaux Arts mansions along 16th Street. Some years later, Totten's son knocked on the Heckers' door and said their apartment was the one the Tottens had lived in.

To call it an apartment is an understatement. In New York it would be a maisonette, in some other places a triplex. Actually, it is a series of attached houses, shaped around an entry court. Each unit has its own front door and interior stair. Most are three stories high, though some have been split into smaller units. A community heating plant keeps the whole building warm, very warm, for a very small maintenance fee.

The Hecker unit has a spacious master bedroom with windows looking down into the living room. A sunroom on this floor serves as a child's bedroom. Two more bedrooms are on the third floor.

Despite the magnificent living room on the first floor, the Heckers felt they needed a different organization of space. The kitchen, like most kitchens built in the 1920s when servants were disappearing from the scene, was tiny, hardly big enough for the cook and the frying pan. The dining room was fair sized, but it also served as the hall to the living room.

"When we had company," said JayEtta Hecker, "all the guests would try to cram themselves in the kitchen, or stand plaintively outside the door. It was almost as bad when it was just family."

The Heckers spent several years talking to designers -- "At one stage I wanted a medieval kitchen". Finally they settled on Constructive Alternatives, headed by John Fleming. The designer, Petter Boe, suggested a fanciful scheme, complete with a sunken floor and a loft. It was estimated at $25,000. The Heckers shouted "too much" and put down their foot at $15,000. Boe and Fleming came back with a simpler plan, but because of unknown problems about the bearing walls and other construction details, they worked on a cost-plus basis.

The final cost was close to $40,000, and the job took from September 1980 to February 1981. The plan was to remove most of the interior walls, and replace them with double supporting arches and a curving D-shaped glass block wall. A J-shaped bar protrudes into the living room from the kitchen. (The "J" and "D" shapes, more apparent on paper than in the room, were not planned to coincide with the owners' initials, they just happened.)

"Petter said we should open up the kitchen to the rest of the house because it's insane to perpetuate the 19th-century idea that food just appears miraculously on the dining table," JayEtta Hecker said.

"It was a very complicated job," said Fleming. "We found there were no supporting walls in the basement. So we had to build new supports there, as well as put in steel beams and supports on the first floor."

But first they had to put in temporary walls to hold the whole thing up while they tore out the old walls.

The glass block wall hides the laundry and pantry. A mirrored cabinet seems to extend it. The appliances line the wall in an almond white color scheme. The floor is quarry tile and so is the counter top.

"We love the counter," said JayEtta Hecker. "I work here all the time." (Both the Heckers are economists. She is a free-lance, he works for the Bureau of Labor Statistics.) The counter (with its comfortable stools) serves the Heckers as a dining table, a study table for adults and children, a place to spy on what's cooking, and in general sort of a house center, a watering trough, a place to meet for family and guests.

The Friends of George Oakley Totten, of which I am a lifetime member, do not look kindly on disrupting his perfect cube room nor intruding a glass block wall into his romantic design. The violations of his grandiose space are hard to forgive.

But the place suits the Heckers way of living just fine, thank you. And if you come to dinner, there's plenty of room for everybody to help.