Although its financing remains blocked by a congressional deadlock, downtown Washington's proposed convention center won formal support yesterday from the National Capital Planning Commission.

The commission approved the proposed site on the south side of New York Avenue between 9th and 11th Streets NW and endorsed the city government's proposed construction program. It advised the city to design the structure with "a quite and distinctive facade, enriching . . . the vista between the White House and Mount Vernon Square." It also recommended that small shops be built on the sides and rear of the structure.

Although the City approved the site early last year, the planning commission was still officially on record as endorsing the nearby site of the abandoned Eisenhower Center project, an earlier version of the now-proposed $110 million convention center. Approval by the commission, a federal body, is legally necessary.

Ben. W. Gilbert, D.C. director of municiple planning and an alternate member of the planning commission, said the action would permit the city to move swiftly to acquire the new site and relocate its residents and businesses, if and when Congress approves funding.

A House-Senate conference committee broke off deliberations on the D.C. budget for fiscal 1978 because of a disagreement over the city's request for $27 million in start-up costs for the project. The House has approved the project, while the Senate has rejected it. Lacking a budget, the city is maintaining operations under a stopgap appropriations resolution.

Because of the Senate's position on funding, Hadley Roff, staff director of the Senate District Subcommittee and an alternate member of the planning commission, abstained from yesterday's otherwise unanimous vote.