Robert M. Bass, one of four young billionaire brothers from Fort Worth, decided last year that he liked Washington. And with increasing business and cultural ventures here, he needed a place to stay when he and his family visited the nation's capital.

So he bought Ulysses S. Grant's 127-year-old summer White House, a three-story, red brick mansion in Georgetown, for nearly $2 million, well more than twice its assessed worth.

But Bass, 37, is a Texan, and he wasn't through dealing. A few days later, he bought the guest house to the rear of the Grant mansion for another $1 million, nearly four times its listed value.

Now, Bass has agreed to buy a third home in the 3200 block of R Street NW, a nine-year-old contemporary next door to the mansion, for about $850,000 to $900,000. In addition, he apparently has sought, unsuccessfully so far, to buy a carriage house, also to the rear of the mansion.

The series of purchases is dwarfed in Washington only by the $6.5 million purchase earlier this year by Sen. John D. Rockefeller IV (D-W.Va.) of a seven-bedroom estate in the Crestwood community near Rock Creek Park. In short order, Bass has collected some of the most expensive homes in the city.

The question is why.

But Robert Bass is not talking about his Washington deals. His family, soaked in the riches of oil and years of savvy stock investments, zealously guards its privacy with strict orders to subordinates that they are not to discuss the clan's dealings.

As a result, Bass' Washington purchases have created an air of mystery, even for people who know him and have been involved with the transactions.

Economist Michael K. Evans, who agreed to sell his contemporary home at 3232 R St. to Bass, said that "Bob came and looked at . . . my home and then had some other people look at it. He came into my office and we were able to close the deal in about an hour."

Evans said that he had a half-dozen real estate brokers appraise the value of his glass-and-brick house, built by architect Sam Reid Dunn in the mid-1970s over the vehement protests of the late Supreme Court Justice Abe Fortas, his wife and other R Street residents who once picketed while it was under construction claiming it was a desecration of the neighborhood. He declined to reveal the selling price, although other sources estimated it at between $850,000 and $900,000, the amount Evans was reportedly asking.

"It was the market value, but nothing spectacular," Evans said. He recalled Bass as saying about the price, " 'That sounds right,' and that was it." He said the settlement on the sale is set for June 17.

Nonetheless, Evans said the multiple Bass purchases on the quiet, tree-lined street a block west of the Dumbarton Oaks museum and nature preserve remain a mystery. "They're very friendly, but they're very close-mouthed," Evans said. "When you start prying a bit, they change the subject."

One acquaintance of Bass said that he and his wife Anne "like Washington, and he has more and more business here." But the acquaintance said that the Basses, who have four children, "don't like living in hotels" and have found it a "real hassle to bring the kids and have them running around hotel hallways.

"Any house in Georgetown is a wonderful investment, and it's nicer to have a home to go to when you're here," the source said.

This acquaintance and others suggested that Bass has purchased the other homes around the Grant mansion as a measure of security, a prime concern for a wealthy family like the Basses.

"You want to have some control over who comes in your front and back door," said one person familiar with the multiple home purchases.

But so far Robert Bass has been unable to buy the home owned by Janice W. Frey, a 49-year-old real estate broker. She has lived since 1975 in a converted workshop, laundry and carriage house that is at the end of a cobblestone driveway immediately east of the mansion.

Frey's rustic, seven-room brick and wood-beamed home, at 3234 R St., borders a cul-de-sac where 11 cars can be parked, an important consideration in congested Georgetown. The District government has assessed its worth at $249,678.

Several sources said that Bass has sought to buy the property but that Frey has refused the overtures. However, another source said Bass has made no efforts to buy the Frey house and that no offer is pending.

Frey said she won't talk about her prospective new neighbors.

"In the spirit of neighborliness and privacy," she said, "I'll remain silent. I love this home. It's never occurred to me to sell it."

While Bass and his family won't be able to live in the mansion for as long as another year while an extensive historic restoration is completed, he already has assumed a typical Georgetown resident's role, protesting the advance of development in the historic community.

Bass signed a Georgetown citizens' petition against a proposal to build a small hotel at Wisconsin Avenue and Reservoir Road, a block away from the mansion that he listed on the petition as his residence.

Bass last year also became one of 40 unpaid board members of the National Trust for Historic Preservation, which is based here, and serves on the collectors' committee of the National Gallery of Art. The art panel is made up of people who have contributed $5,000 annually to the gallery and meets once a year to advise gallery officials on art purchases.

The Bass brothers, with their vast fortune estimated by Newsweek magazine as approaching $4 billion, are often affected sharply by various regulatory and congressional actions here. The brothers, who do much of their business through a corporation called Bass Bros. Enterprises, sought last year to split some of their joint holdings into companies controlled by the brothers individually and to do it free of taxes, which is allowable under federal law. The brothers also wanted to keep their oil production in a joint company, but the Treasury Department balked at the arrangement.

At the brothers' request, Sen. Lloyd Bentsen (D-Texas), a longtime Bass family friend and financial beneficiary of the Basses' campaign war chests, said he sought to amend federal law to allow the brothers to sever some of their corporate ties. But he dropped his request when the Treasury continued its opposition. He said the Basses subsequently appealed their case to the Internal Revenue Service.

Robert Bass made an initial foray into the Washington development world last year, buying half interest in a prospective office building project near Tysons Corner. Tom Georgelas, one of the partners in the venture, said Bass put up a deposit on the project and may have eventually had as large as a $7 million interest in the three six-story buildings.

But Georgelas said that Bass, for "no particular reason," sold his interest last November to Rozansky & Kay, a Washington development company, but "made no money" on the deal. Georgelas said Bass "just decided it wasn't the kind of deal he wanted to do," but offered to stay in if no one could be found to buy him out.

So, for the moment anyway, Bass' major Washington venture is the restoration of the Grant mansion, the 11-room centerpiece of his purchases. In addition to Grant's summertime residency there more than a century ago, the white-columned structure was home in the 1930s to a group of young White House advisers like Thomas G. (Tommy the Cork) Corcoran and Benjamin Cohen, who came to be known as Franklin D. Roosevelt's New Deal Brain Trust.

The mansion, the guest house, the carriage house and the acre of land surrounding them were part of the same property until architect Dunn bought all of it in 1973 for $360,000 and then subdivided it. Dunn remembers it as "a time when properties like that were thought of as not desirable." He subsequently sold off the homes and built, after a long public dispute, the contemporary home that he lived in from 1976 to 1979.

In buying three of the properties, Bass has, in effect, reassembled much of the tract as it used to be. He has hired an architect, David Schwarz, to oversee the refurbishing of the mansion. Schwarz declined to discuss the work, saying his client had asked him to be silent.

David Morgan, a contractor who specializes in historic renovation work and has been hired to work on the restoration, was less reticent.

"The intent is to keep its historical nature," Morgan said of the mansion. "If we have to take out something, we'll replace it. There are certain parts that have worn out, such as termite damage in the ground wood floors. We don't have the total picture yet."

Morgan said that work crews have gutted the bathrooms, taken down some plaster ceilings and walls and ripped out all the kitchen facilities. There will be no changes to the exterior of the home, except repairs, he said.

Morgan said he has no idea what the restoration, under way for two months and expected to take another year, will cost.

Such attention to detail and a sense of history pleases at least one neighbor, attorney Carolyn E. Agger, Fortas' widow. Agger still views the contemporary home with disdain, saying that it "basically ruins the land." But Agger said she welcomes the Basses' entry into the neighborhood.

"I understand they're very wealthy people," she said, "and that's good because they won't try to squeeze anything more out of the property."