More private property than previously announced will be needed to accommodate a long-planned widening of a 10.5-mile stretch of the Pennsylvania Turnpike's Northeast Extension.

Additional 10- to 25-foot-wide swaths are needed to comply with new regulations governing storm-water runoff. Many homeowners along the route have only recently learned of the change.

The project will affect property owners in the Montgomery County communities of Plymouth, Whitpain, Worcester, Upper Gwynedd and Towamencin Townships.

The work will run from Exit 20 at the Mid-County interchange in Plymouth Meeting to Exit 31, the Lansdale interchange.

Turnpike Commission spokesman Carl Defebo said the agency would be hold an open house this evening at Pennbrook Middle School in North Wales.

Whitpain Supervisor Joe Palmer has a word of advice.

"If you have property bordering the turnpike, go to the meeting," he said. "Find out how it affects you."

At a similar session on March 29 that attracted more than 300 people, Palmer said many property owners were astonished to learn that an additional 10 to 25 feet of their property could be lost to the project, contrary to preliminary plans they had seen several years earlier. And in many areas, sound barriers had been eliminated as well, he said.

"When we went there, we were very surprised at what we saw," Palmer said. Three township homes will be taken, and parts of dozens of other properties, he said. The worst case involved a business owner who faces the loss of one-third of his property, Palmer said.

For the entire stretch, Defebo said, nine homes will be taken; three are vacant. Also, 150 property owners will lose part of their land. All will be reimbursed at market value, he said.

Defebo said the expansion, which has been under consideration since 1999, is needed to handle the ever-increasing volume on the Northeast Extension, now up to 66,000 vehicles a day.

"This is the most heavily traveled four-lane stretch of turnpike in the state," he said. "With that volume of traffic, this is a stretch that is just crying out to be widened."

The Turnpike Commission is proposing to add an additional 12-foot traffic lane in each direction, increase the width of the median from four feet to 26 feet, and widen the shoulders from 10 feet to 12 feet.

The new storm-water rules require more open space to allow water runoff to sink into the ground instead of draining it away in pipes.

From shoulder to shoulder, the project will add 50 feet to the roadway, Defebo said. While the turnpike owns a total of 200 feet including the roadway, shoulders and right of way, in most cases that is not enough to meet modern standards for highway design, he said.

Costs for the expansion range from $250 million to $350 million, making it one of the most expensive projects in turnpike history, he said. Work could start next year on bridges, but the bulk of the work would not start until 2011, he said.

In the coming weeks, area legislators and local officials will be meeting with PennDot and the state Department of Environmental Protection to look at ways of addressing the storm-water problems and lessen the impact on property owners.

"We are hoping we can influence the design of the road," said State Rep. Kate Harper (R., Montgomery). "We are trying to come up with reasonable alternatives.

Palmer said that last week, the Whitpain supervisors unanimously passed a resolution that rejected the current plans and called upon the Turnpike Commission to look at the overall project with compassion for neighborhoods.

"I think everybody realizes there needs to be an expansion," he said. "They can get this done in a responsible manner that satisfies their goals with a minimal impact on the community."

But not everybody is upset, said State Rep. Mike Gerber (D., Montgomery). "We have some constituents who have known about it for seven or eight years and they can't wait," he said. "They will get a good value for their homes and moving costs paid for."

Tonight's session will be from 6 to 9 p.m. Pennbrook Middle School is at 1201 E. Walnut St.

Large maps will detail the properties that could be involved, and engineers and designers will be on hand to answer questions, he said.

Contact staff writer Nancy Petersen at 610-701-7602 or npetersen@phillynews.com.