The House of Representatives approved yesterday a $26 million garden wall memorial to Franklin Delano Roosevelt, to be built around the western edge of the Tidal Basin--the first major monument to this nation's longest serving president.

The memorial legislation, approved by the Senate in March, now goes to President Reagan, who has frequently praised Roosevelt but had not indicated as of yesterday whether he would sign the bill.

There is currently one modest monument to FDR in Washington--a small block of marble placed opposite the National Archives in 1965, 20 years after Roosevelt's death. Approval of the new memorial comes during the centennial of Roosevelt's birth in 1882.

Former New York representative Eugene J. Keogh, chairman of the 27-year-old FDR Commission, said yesterday that if Reagan signs the authorizing legislation, the commission will ask Congress to appropriate funds to build the memorial over a three-year period beginning in 1984.

The memorial, designed by San Francisco landscape architect Lawrence Halprin, will comprise an 800-foot-long, 14-foot-high garden wall, with waterfalls, pools, gardens and sculpture. It would wind around the cherry trees that ring the western edge of the Tidal Basin and include 27 acres of what is now West Potomac Park.

The road and playing fields there now would be relocated.

It is a smaller version of a $50 million FDR memorial proposed in 1978 and criticized by then-secretary of the Interior Cecil D. Andrus as extravagantly expensive to build and maintain.

Congress took no action on that proposal, as it similarly failed to authorize two previous widely criticized FDR memorial designs.

The earlier design had called for a 1,200-foot-long garden wall and a large visitors' center and would have cost at least $1.2 million a year to maintain, according to National Park Service estimates.

Andrus has said it would make an inappropriate tribute to a president "who made it very plain that he did not want a large, costly monument erected in his honor in Washington."

Before his death, Roosevelt reportedly told Supreme Court justice Felix Frankfurter that he wanted a small memorial no bigger than his desk.

The plain block of marble at Pennsylvania Avenue and Ninth Street NW, paid for with private contributions, is desk size.

The "modified" design, which has been approved by the Fine Arts Commission, shortens the wall and eliminates the visitors center.

It will cost $763,000 a year to maintain, according to National Park Service estimates.

The Congressional Budget Office, however, estimates the modified memorial will cost about $1 million to maintain.

The CBO also estimates it will cost more to build than its sponsors say--$31 million, according to the CBO, instead of $26 million.

Congressional approval of the memorial came as the existence of the FDR Commission itself seemed threatened.

While yesterday's House vote in favor of the memorial was 254 to 151, last September the House voted by a margin of only 15 votes to keep the memorial commission alive for another year.

Like the various memorial designs, the commission itself has been frequently criticized.

In 1955 Congress chartered the commission, composed of four congressmen, four senators and four private citizens, and in 1959 it authorized the 27-acre West Potomac Park site for the memorial.

The commission has spent more than $500,000 in its attempt to find a memorial that would be approved by Congress, the Fine Arts Commission and the National Capital Planning Commission.

The planning commission has yet to formally approve the latest memorial plan, but informally reviewed it last year and found it "excellent," commission Chairman Helen M. Scharf told a congressional committee last fall.

Sen. Mark O. Hatfield (R-Ore.), a member of the commission and the memorial's chief Senate sponsor, called yesterday "a happy day" and added that Congress will not be asked to fund the memorial for at least two years, "when an improved budgetary situation will make possible actual funding of the memorial."

A major portion of the estimated $26 million cost of the memorial will be for underground pilings and supports for the heavy stone walls, fountains and pools, since West Potomac Park is filled-in marshland, or "mud flats" as Thomas Jefferson called the area. Both the Lincoln and Jefferson memorials are built on huge underground concrete pilings.

The major impact of the memorial will be on traffic patterns in West Potomac Park, where a portion of the present road ringing the Tidal Basin will be eliminated and traffic moved onto a widened Ohio Drive, along the river's edge between the Tidal Basin and Lincoln Memorial.