The U.S. Court of Appeals here has at least temporarily blocked the demolitin of the 12-story Munsey Building, a graying structure located on a site just off Pennsylvania Avenue NW where the Marriott Corp. hopes to build an 800-room hotel.

Lawyers for Don't Tear It Down, a local preservation group, won the order last week from Circuit Judges Edward A. Tamm and Abner J. Mikva after a lower court judge had ruled that the Munsey's owner, the Pennsylvania Avenue Development Corp. (PADC), could proceed with the building's destruction.

If the preservationists' suit is eventually successful, it would stop construction of a key portion of the redevelopment of the Pennsylvania Avenue area. For the moment, however, officials of PADC and Quadrangle Development Corp., the prospective builders of the Marriott complex, say they are concerned the suit will delay the scheduled start of construction next summer.

In addition to the hotel, Quadrangle and Marriott plan to build an office building with 500,000 square feet of space and a tri-level retail shopping complex, all of it in a block bounded by 13th, 14th, E and F streets.

PADC's plans call for leveling the entire existing block, with the exception of the National Theatre and the National Press Building.

The dispute over the Munsey Building, which until recently housed D.C. government and private offices, centers on a provision in the Pennsylvania Avenue plan that says the Munsey and other buildings, "could remain as long as their owners chose not to redevelop them."

PADC officials have taken that to mean that since they bought the building earlier this year for about $4 million they can tear it down if they wish to make way for the Marriott-Quadrangle redevelopment work.

As a result, PADC officials routinely applied for a demolition permit from the city and District officials, according to court papers, were about to approve the permit in late September when Don't Tear It Down filed the suit.

Don't Tear It Down claims, among other things, that PADC and city officials have violated that D.C. Historic Protection Act by not submitting the demolition permit application to the city's Joint Committee on Landmarks.

Under the Historic Protection Act, demolition of buildings in historic districts as in the case of the Munsey, must be reviewed by the Joint Committee.

"It is evident . . . that the (Pennsylvania Avenue) plan does not require the demolition of the Munsey Building," Don't Tear It Down's attorneys, S. Mark Tuller and Phyllis Cohen, have argued in the case.

"Thus, the denial of a demolition permit would be as compatible with the (avenue) plan as would the issuance of such a permit, as the plan allows for either demolition or retention of the structure."

The Munsey is apparently of little historical significance, with the avenue plan merely calling it a "well-defined, early 20th Century commercial building."

Tuller said that one main reason for filing the suit is to "preserve the (review) process" before a demolition permit is granted.

But U.S. District Court Judge Aubrey E. Robinson Jr. ruled two weeks ago that PADC has substantially complied with various requirements and that the city's "interpretation of its responsibilities (under the Historic Protection Act) . . . is reasonable."

It is that ruling that Don't Tear It Down is appealing.

Peter Meszoly, PADC's general counsel, said construction of the Marriott complex probably would not be delayed if PADC wins the case by early January. He said PADC has asked Justice Department lawyers handling the case to seek an expedited appeals court hearing.