Plans to double the size of Highcliff Elementary School won preliminary approval last night from the Ross Planning Commission.

Commission members had questions about storm water control, traffic and parking, and North Hills School District officials promised answers when they return next month to seek site-plan approval for the $18 million project.

The planning commission is an advisory body and township commissioners will have final say on the project. About 50 residents attended the planners' meet at the Ross municipal center.

The proposal to increase the capacity of Highcliff from about 200 to 520 students is part of a multiphase plan to have four approximately equal-sized elementary schools in the district.

The plans presented last night would maintain much of the existing wooded areas around the school. The district's proposal also would require much less earth-moving and disturbance of hillsides than an earlier plan. That plan required a variance from the township's steep-slope ordinance, a request that was rejected by the zoning hearing board.

The new plan calls for most of the parking for the expanded school to be located on property across Peony Avenue that is owned by Christ Lutheran Church. The school district and the church have agreed on terms of a 50-year lease for parking rights.

Daniel Goodwin, the planning commission's consulting engineer, told commission members that if the district's calculations of storm-water flows prove correct, downstream property owners should face fewer flooding problems.

School district Superintendent Joseph Goodnack said the planners' preliminary approval has kept the project on track for groundbreaking this summer. Construction would take about a year, he estimated, with students returning to the building for the start of school in fall 2010.