South Fayette School District could end up with another building on its Old Oakdale Road campus, depending on how school officials decide to deal with a growing population of elementary pupils.

The primary school houses more pupils than any other in Allegheny County -- about 893 in kindergarten through fourth grade. As the township continues to attract more residents over the next decade, enrollment is expected to rise 20 percent, to 1,072 students.

School board members are weighing the benefits of adding to the building versus constructing a new school. One way or another, the district must increase its space.

This year, five portable classrooms adjacent to the building have accommodated the overflow. But board President William Newcomer said adding trailers is not a long-term solution.

School board members are considering two options.

After completing a facilities study, Eckles Architecture & Engineering, of New Castle, recently recommended constructing a new intermediate school for grades three to five and renovating the primary school for kindergarten through second grade.

If the fifth grade is moved into a new intermediate school, more space would be available for grades six through eight in the middle school, where enrollment is expected to increase by 55 percent by 2017.

The second option is to renovate and expand the elementary school. In this scenario, some pupils might have to be moved into trailers or relocated to other areas of the building during the work.

The recommended plan -- constructing a new building and remodeling the existing one -- would cost about $47 million, versus about $37 million for the other option.

Despite the higher price tag, Mr. Newcomer said the recommended option appeared to be the best choice. One reason was that renovations to the 35-year-old building would have to be extensive and cost more than originally thought.

School directors want more information. In April, the board authorized Eckles to do a second study to determine the long-term costs of operating an additional building versus operating a single, larger one.

After the study is finished in June, school directors will use it to help determine a course of action. Mr. Newcomer hoped to decide by fall.

Depending on the work schedule and financing, new space could be ready for occupancy in two to four years.

Starting work sooner may require a referendum to borrow money or increase the property tax rate above the normal limit.

If the district instead waits for its borrowing power to increase -- by paying off older bonds and restructuring existing debt -- a referendum would not be needed but the crowding could worsen.

Now, the elementary school is running out of space in the gym, rest rooms and cafeteria. Next year, some courses, such as art and music, may travel from room to room on a cart because of a lack of space. After next year, trailers would have to be added at a rate of two per year.

According to a demographic study completed in November 2007 by Shelby Stewman, a Carnegie Mellon University professor and principal of Stewman Demographics, 179 more students will enter the elementary school by 2017.

The Eckles study, completed in March, found the elementary school was too small and in the worst overall condition of all the district's buildings.