Adding momentum to the pace of development along Pennsylvania Avenue, a Boston builder has agreed to buy half a block of land on the avenue between 12 and 13th streets NW, and build a 16-story office building.

Cabot, Cabot & Forbes Inc. of Boston plans to put up a luxury office building, faced with granite, where there now are vacant lots, empty buildings and parking garages.

Expected to cost about $25 million, the project will extend eastward by another block the development frontier that is moving across downtown Washington.

The latest project means that almost the entire three blocks on the north side of Pennsylvania Avenue from 12th to 15th streets NW will be rebuilt in the next five years.

"Pennsylvania Avenue will become the leading business avenue in downtown Washington in the decade to come," Gerald W. Blakely Jr., chairman of Cobot, Cobot & Forbes, said in a statement yesterday.

W. Anderson Barnes, executive director of the Pennsylvania Avenue Development Corporation (PADC), said he expects private developers to announce plans within the next two years for rebuilding all the blocks west of the FBI buildings beginning at Ninth street.

The Cabot, Cabot & Forbes building will be designed by the Washington office of architects Skidmore, Owings and Merrill and will have "ground floor uses that create livelihood and action" to draw people into the neighborhood at nights and on weekends.

The 400,000-square-foot building will occupy all of the block bounded by 12th, 13th, and E streets and Pennsylvania Avenue, except for the office building at the corner of 13th and Pennsylvania.

Across 13th Street, Quadrangle Development Corp. is building a $17 million office building and plans to rebuild the rest of the block facing Pennsylvania between 13th and 14th. PADC last month picked Quadrangle and Marriott Corp. to build a $110 million hotel-office complex that will use the remainder of that block except for the National Theatre and National Press Building.

And on the block between 13th and 15th streets, is the Willard Hotel, which nine developers are bidding to rebuild and reopen. Before Christmas the PADC is expected to choose one of those firms to renovate the Willard and develop the land next to it, a $40 to $50 million project.

Pennsylvania Avenue itself is now being rebuilt along those blocks as part of a PADC-financed $14 million project that will create a new plaza and park and reroute traffic.

Retired Air Force general Elwood (Pete) Quesada, who has been chairman of the PADC since it was founded, stepped down last wek at the completion of his six-year term. The new chairman is Joseph Danzansky, chairman of the Giant Food supermarket chain.

The federally financed PADC is overseeing redevelopment of about 20 blocks along the north side of the avenue between the White House and the Capital. It is putting in better streets, side-walks and other improvements and buying land assembling land for private development of several major developments.

The Cabot, Cabot & Forbes building, however, is a private venture. The developers have signed an agreement to buy their land from members of the Christian Heurich family, who once ran a prominent Washington brewery and still own a lot of downtown land.

The purchase price has not been disclosed because the sale has not been completed, but the developers say they hope to sign the deal, clear the land and begin construction by early next year so the building can be open by 1981.

Blakely said the Boston firm, which has not previously worked in Washington, decided to build the downtown office building "as a result of the early acceptance of the PADC and its efforts to renew the downtown."

No major tenant has been signed up for the building, indicating the developers are confident the downtown office market is strong enough to support a speculatively built building.

The building will be 160 feet tall - equivalent to 16 stories - but as a result of height restrictions just imposed by the PADC will be 18 feet shorter than structures on the blocks between 13th and 15th streets.

District of Columbia zoning regulaions limit building heights to 120 feet except for the five blocks facing Pennsylvania Avenue between 10th and 15th streets.

Saying the taller buildings - which produce more income for owners - were needed to spur construction along the avenue, the city last year raised the limit on those blocks to 160 feet, plus an additional 18 feet for rooftop structures.

But last month - in action not made public until now - the PADC's board of directors adopted rules prohibiting rooftop structures on the three blocks between 10th and 13th streets.

The PADC has authority to impose strict controls in the area it is helping redevelop, and ruled out rooftop structures on those blocks to ensure that jutting elevator towers and ventilators would not mar the view of the avenue's new skyline. CAPTION: Picture, Sixteen story, $25 million office building is to be constructed by a Boston firm on this site between 12th and 13th streets NW on Pennsylvania Avenue. By Margaret Thomas - The Washington Post