Construction has started on the Washington Center Office Building, the second phase of development at Washington Center, a full-block, 1.3 million-square-foot project at 1001 G St. NW. The Washington Center Office Building will adjoin the 950-room Grand Hyatt Hotel that was completed in June.

Also included in the $250 million project is the renovation of the turn-of-the-century McLachlen Building next door.

That building will remain separate but will share a common lobby with the Washington Center Building.

The 12-story, 330,000-square-foot Washington Center Building is being developed by Quadrangle Development Corp. and was designed by RTKL Architects.

It will feature a street-level concourse and 20,000 square feet of retail space. Other amenities include a health club and a lap pool.

The building also is expected to have child-care facilities.

A Quadrangle spokeswoman said child care is "somewhat rare in a broadly leased building like this."

The building will also have a full-height atrium with a landscaped court, waterfalls, pools, marble floors and walls, artwork, mahogany paneling, balconies and terraces.

In Washington, atriums have grown in popularity as height limits in the District reduce the potential for good views.

"Atriums allow you to develop a building with a special kind of interior view, and you don't have to worry about what the street looks like on the outside," said Christopher Gladstone, a vice president of Quadrangle.

Five levels of parking will be available under the building.

Omni Construction is general contractor for the project.

Completion is expected in the spring of 1989.