The Montgomery County Planning Board yesterday turned down a proposal to classify the Washington Adventist Hospital's aging sanitarium as an historic structure, a decision that imperils efforts by Takoma Park groups to save the building from pending demolition.

The board voted 4-to-1 that the hospital's ability to provide medical care constitutes a stronger public interest than preserving the 75-year-old building around which Takoma Park developed. Hospital lawyers had argued at a Nov. 16 hearing that the sanitarium would be too costly to restore and that its presence would block future buildng expansion.

The board acknowleged that the sanitarium meets the standards for designation as an historic structure. But the majority agreed with board chairman Norman L. Christeller, who said the hospital's expansion plans shouldn't be encumbered by lengthy administrative procedures that such a classification would require for any building alterations.

Community groups had hoped the delaying procedures would force hospital officials to examine alternative uses for the sanitarium building.

The decision drew a complaint from Takoma Park City Attorney Thomas Gagliardo, who said the planning board "clearly exceeded its authority" by considering issues other than the sanitarium's historical significance.

Officials of the county Department of Environmental Protection are withholding a decision on whether to grant the hospital a demolition permit until after Dec.2, when the county Circuit Court is scheduled to consider issuing the permit.

Hospital officials yesterday said they consider recent architectural proposals for the sanitarium unfeasible, and will continue to press for its demolition.