When the popular SportsWorks complex at the Carnegie Science Center closed in August to make way for the light rail extension to the North Shore, officials stressed that it wouldn't be sidelined for long.

Yesterday, the city planning commission took a key step in facilitating its return.

In a unanimous vote, the commission approved plans for the new SportsWorks building, clearing the way for construction to begin early next month on the $5 million project.

The new 25,000-square-foot structure will be built adjacent to the science center in what is now parking lot space. It will feature a main exhibit area as well as a mezzanine that will double as a lobby and house some classroom space.

Although the new complex will be slightly smaller than the old building, Ann M. Metzger, the science center's acting co-director, said it will be large enough to house many old exhibits as well as some new ones.

That means visitors still will be able to measure the speed of their fastballs, climb a rock wall or bounce on a trampoline as part of the many exhibits.

Some of the new attractions will focus on health, nutrition and lifestyle. They will involve "some cool things that have to do with educating about the human body, about nutrition, about activity, how we burn calories, that type of thing," Ms. Metzger said.

The new building will feature a pre-engineered metal skin as well as a wall of glass facing Heinz Field. Science center officials hope to open it on Labor Day 2009, roughly a year after the exhibits shut down.

As part of a 10-year master plan, a 10,000-square-foot reception center with a ticket area and expanded gift shop eventually will be built to connect SportsWorks with the science center.

The Port Authority plans to demolish the old SportsWorks home, in the former Miller Printing Co. building, to clear a path for a light rail stop. The demolition will free up about 225 parking spaces for science center use by the end of the year.

Also yesterday, the planning commission, in a 4-2 vote, recommended against designating the 155-year-old St. Mary's Academy building in Lawrenceville as a city historic structure.

The commission determined that the building, on 46th Street, was a "religious structure," meaning that only its owner could nominate it for designation. The decision was a victory for the Catholic Cemeteries Association, which wants to demolish the building and use the lot as green space and cemetery property.

After learning of the proposed demolition, Keith Cochran, a Lawrenceville architect, nominated the building for city historic status in an effort to prevent its demise. Joe Huber, an attorney and director of family services for the Catholic Cemeteries Association, said officials are willing to consider a sale of the building if a new owner will maintain it. He said the association doesn't have the financial resources to do so.

City Council now will take up the proposed designation.