Robert L. Alexander, turned down by 50 lending institutions before he built his Northeast Washington fast-food restaurant two years ago, only had to knock once to sell his idea for an $800,000 office building near a former riot corridor.

Just months after completing his Stadium Hardee's fast-food restaurant on Benning Road, the 34-year-old developer picked up his lance and took another charge at the lending institutions.

This time he successfully found financing for a two-story medical office building.

His plans were so promising,he said, that he received immediate approval for permanent financing from Washington Federal Savings and Loan. And officials were apparently so confident about the project they leased part of the first floor for a branch office.

Alexander's plans hinged on the decision of John Hechinger Sr. to build a $13.3 million mall directly across the street from Alexander's proposed office site.

And it wasn't luck that brought Alexander to 1647 Benning RD. NE: "I monitored (Hechinger's) interest in the convention center and the mall. You see he, too, was dreaming and planning. So with my own idea, I started construction three months before he did."

Alexander said he began trying to put the project together in the fall of 1978. He used a $5,000 retainer to hold the land that cost $150,000 in the spring of 1978, while he negotiated with Riggs Bank for a construction loan.

During that period, Alexander said the land's value doubled because of Hechinger's plan for the mall.

As Alexander spoke, his voice was filled with the energy of a salesman and with his obvious excitement about the project: "There are no medical offices privately owned within a square mile and there is no pediatrician even farther. And the closest banking facility is on H Street. After that you have to travel a mile to find some place to put your money."

"He does bust his butt," explained Jim Harris, the president of Washington Federal Savings and Loan, who funded both the Hardee's restaurant and the new office building. He said Alexander does not seem to give up when he is involved in a project.

The savings and loan president said Alexander made a few mistakes that ultimately caused some delays and higher costs in the project, but was still able to build the office complex with very little knowledge about construction.

"I think he is like all small developers. He has had a hard row to hoe. He did not have much available cash so he had to go to the Small Business Administration and get guarantees," said Harris.

Alexander, who lives in a Southeast townhouse with his wife and 14-month-old son, said he immediately began working on the office building because "prices are going up by the day, the hour and the minute."

He said he later discovered a number of people were interested in the site planned for his office building: "While they were doing their research, I moved on it."

Getting financing this time proved a great deal easier than before, but Alexander said he was delayed during negotiations with Riggs Bank for a construction loan. At one point, he said, he had to go back for an additional secondary loan of $100,000 as he attempted to finalize the deal.

When interviewed two years ago for a story about the opening of the Hardee's restaurant, Alexander drove a Mercedes and wore a fashionable suit, white shirt and tie.

When interviewed last week, he was driving a 1976 Dodge Colt and wearing a dirty jacket. "I have had to juggle my finances to make this project work," he said, noting that he still owns a 1965 Rolls Royce. He said he had to refinance his home to help meet about 20 percent of the project's total cost.

Alexander, who is the developer of the project, has three partners. They are John V. Golden Jr., a radiologist; Wilbur Callender, an obstetrician and gynecologist; and Cyril Brown, a dentist.

The two-story office building will house 12 doctors and dentists, and include a drive-thru pharmacy as well as the savings and loan branch office.

Last weekend, Alexander began working day and night to meet the March 1 deadline for completion of the office building. Workmen were marking out space for the doctor's office, and Alexander was urging them to work at the fastest pace possible.

Alexander, a native of Florida who holds a Ph.D. degree from Rutgers University, was an aide to President Richard Nixon. He later worked for the Central Intelligence Agency as a minority recruiter.

The black developer, who said he is considering adding a third floor to his office building, said he wanted to make sure everyone understands that he is only a small developer.

"My little $800,000 project is only a drop in the bucket. What the hell is $800,000 when millions of dollars have been spent for construction in the city?" he said.

Alexander recently changed the name of his Hardee's restaurant to Stadium Hamburgers to eliminate franchise costs. About four months after completing his restaurant in 1978, Alexander attempted to open another Hardee's downtown, but could not come up with the required $250,000. Lending institutions told him his restaurant, at 2301 Benning Road, had not been operating long enough to develop a track record.

Asked if he planned to be involved in any more Washington development projects, he said, "After this, I am going to begin developing my family and try to live off the building's profits."