The Federal Fine Arts Commission yesterday approved a scaled-down proposal for a luxury condominium building overlooking Rock Creek Park behind the Shoreham Hotel.

With the last major governmental obstacle to construction cleared, New York real estate magnate William Zeckendorf Jr. promptly announced plans to start building the $35-million project next spring. He said the first of the 212 units, costing between $200,000 and $225,000, would be ready for occupancy in fall 1982.

The commission unanimously approved Zeckendorf's plans after he sliced a floor out of the proposed building so that it would be less obtrusive to people looking at it up a hill in Rock Creek Park.

As a result David Childs, the project architect, said that the visible part of the structure would be a maximum of 19 feet high on the steeply sloping park side. The building, he said, would not be seen from the park during the summer when the trees are in full foliage but it would be some what visible in winter. On the Calvert Street NW side, the building would be 80 feet high, 10 feet shorter than the Shoreham.

Nonetheless, residents who live near the project still objected to the height, noting that it actually will be 127 feet tall, including the underground portion.

"It's going to be another Rosslyn on Connecticut Avenue," Alice Bralove, one nearby neighbor, complained to the commission.

Jacques B. DePuy, an attorney representing the neighbors, described the condominum as "the sort of building that's located only downtown."

J. Carter Brown, chairman of the commission and director of the National Gallery of Art, said the panel was "sympathetic to the views of the citizens. g

"But they, too have bought their properties on a caveat emptor ('Let the buyer beware') basis," Brown said. "The building is large, but small by the standard of other buildings already there."

He described the condominium, with it's terraced, landscaped facade, as "a handsome effort . . . on balance a net improvement" for the neighborhood.

The only conditions that the commission imposed on its approval were that the National Park Service closely monitor construction of the condominium so that no damage is done to the forest between the building and Rock Creek Parkway and that "great attention be paid to the landscaping on the terraces."

Childs and Zeckendorf said they had no problems with those conditions since they already have been consulting with the Park Service about planned construction work. The Park Service had earlier objected to the condominium, but supported it when the height was reduced and a fieldstone retaining wall proposed for the side of the condominium abutting the park.

Zeckendorf said that with the Fine Arts approval he could now secure construction financing for the condominiums at a time when interest rates on loans are dropping markedly from their record highs earlier this year.

He said the one- and two-bedroom units would average 1,200 square feet in size. An existing 108-room motor inn to the rear of the Shoreham will be razed to make room for the condominium.

The developer, long know for major development projects in New York, said he plans to use the profits from the sale of the condominiums to help finance the $15-million to $20-million renovation of the Shoreham, now scheduled to start in August.

"We realized that when we were buying the hotel (for $35 million) that it had seen its better days," Zeckendorf told the commission. "The Shoreham was a dying hotel."

He said that over a three-year period, all of the hotel's 675 rooms will be redecorated, as well as the lobby and ballrooms. As the renovation is completed, room rates at the hotel will be increased from the current average of about $50 a night to about $80, with suites going for up to $135.