In the 2800 block of P Street NW, a quiet oasis a short distance from the congestion of commercial Georgetown, the stone fences are topped by rifle barrels made for the combatants in the War of 1812.

But it's daggers the neighbors are throwing at each other.

For 10 months now, a small band of residents on the block, including Alice Acheson, the 89-year-old widow of Dean Acheson, President Truman's secretary of state, has been waging war over a plan of one of their neighbors, civil engineer Tom Elmore, to sell his tree-filled side lot for $300,000 so a builder can build another town house. Elmore wants the money so he can expand his own home at 2811 P St.

But in the view of Acheson and others on the block, the new construction would destroy a piece of greenery they say is to be cherished as part of an historic neighborhood, not simply a piece of land to be dealt with like some suburban tract.

So some of the P Street combatants trooped again yesterday to the Commission of Fine Arts, the federal arbiter of architectural aesthetics in Georgetown and much of Washington and the agency that has refereed the dispute.

Twice before, builder Michael Minkoff, who has agreed to buy Elmore's lot, submitted designs for a traditional town house on the lot. But the commission promptly rejected them as out of line for P Street.

Yesterday, Minkoff was back again, this time with a new design that would push the $1.2 million three-story town house to the rear of the property, with a, glass-covered swimming pool in the front, well off the street so the neighbors would not have to see as much of it and also so it would not block as much of the sunlight that pours into Acheson's walled-in garden next door to the vacant Elmore lot.

David Briggs, Minkoff's lawyer, told the commission that the design "preserves the integrity of the Elmore property. . . . It responds to that open space requirement for Mrs. Acheson's garden."

But the neighbors were hardly convinced. Educational consultant Anne Childs, who lives across P Street from Acheson and the Elmore property, said the latest proposal "looks like a shoe box. It's just been slipped back further."

Acheson, who has lived in her home at 2805 P St. for 62 years, is recuperating out of town from recent surgery. But she sent a letter for Childs to read to the commission.

"Very obviously, if this building comes to pass, my garden would become a dark hole, with the trees and shrubs killed or cut down," Acheson said.

The commission quickly and unanimously agreed.

Chairman J. Carter Brown, director of the National Gallery of Art, said the latest design was better than the previous ones. Nonetheless, Brown said the new house "jumps the scale of the ensemble" of pre-Civil War homes on the block.

Minkoff declined to say what his next move would be.

Nor was it clear what Elmore would do. When the Fine Arts Commission rejected the first town house design last October, within days he cut down a large evergreen at the front of his side lot that his neighbors said they liked.