If Lawrence is going to add 20,000 residents south of the Wakarusa River, Douglas County commissioners figure the county’s development regulations may as well be ready to handle them all.

But just how far south those new urban-oriented rules should reach — covering 15,000 acres or 21,500 acres — remains an open question, commissioners discovered Wednesday night.

“The urban growth area’s going to be expanded,” Commissioner Jere McElhaney said. “We just have to make sure it’s reasonable.”

Wednesday’s discussion came as commissioners prepare to redefine Lawrence’s urban growth area, the rural area expected to be absorbed by the city in the next 17 years.

The growth area’s current southern boundary stops at the Wakarusa River, but planners say the area needs to be expanded to accommodate increasing demand for new homes and other development south of the river.

Properties included in the growth area face tighter development restrictions than typical rural properties. Among the biggest effects: Owners no longer would be able to split their land into five-acre lots for rural development without first agreeing to set aside land for roads, sewers and other utilities.

In exchange, Lawrence would endeavor to provide city services to the area within the near future. In this case, Lawrence would be expected to build a new sewage treatment plant on the river by 2011, allowing the area south of the river to soak up thousands of new residents.

Where the regulatory line should stop remains up for debate. Commissioner Charles Jones said he would support going either to North 1000 Road or North 900 Road, which is a mile farther south. Bob Johnson, commission chairman, tentatively backs going with North 1000 Road.

McElhaney wants to crunch the city’s growth projections before backing either one. If anything, he said, the area should shrink.

Commissioners will discuss the issue with Lawrence city commissioners during a joint study session, set for 9 a.m. Oct. 16 at Lawrence City Hall, Sixth and Massachusetts streets.