Fairfax County this week gave Potomac Investment Associates the green light for a plan to build 1,800 residential units along the Penderbrook Golf Course in the Fair Oaks area.

Approval of the project came only after the developer agreed to go along with a last-minute arrangement binding Potomac Investment Associates to "take necessary precautions to make sure wells and septic fields" in the nearby Fairfax Farms subdivision are not hurt by the construction.

Attorney Martin Walsh and county staff members negotiated the agreement while members of the Board of Supervisors debated the merits of the proposal.

The housing units will be built in nine clusters around the existing Penderbrook Golf Course, which will continue to operate, Walsh said.

Residents of Fairfax Farms testified generally in favor of the proposal, but several also said they were concerned about potential water runoff and flooding and long-range damage to wells and septic fields.

The Penderbrook site is on the northwest side of Lee Jackson Highway (Rte. 50) and West Ox Road, north of Fair Oaks Mall and southeast of the interchange of Rtes. 50 and I-66.

The land was zoned residential, for one house per acre; the new plan calls for a PDH-8, planned development housing project with eight units per acre.

In addition to Fairfax Farms, the site is near Fair Oaks Estates and the Navy-Vale community.

"This represents a considerable assemblage of properties by Potomac Investment Associates," Walsh said. "One of the primary objectives of the Penderbrook proposal has been the preservation of the golf course, which is in good condition on rolling terrain."

Some of the holes on the course will be moved slightly to accommodate the town houses, but Walsh said the changes represent improvements to the course itself.

Saving the golf course has been a major objective of the county's land-use plan for the area around Fair Oaks. It is owned and operated by Lynch Limited Partnership. That group will continue to own and operate the golf course, lawyers said.

The land in the golf course was counted as part of the land required to get the county to approve the density in Potomac Investment Associates' project.

There are 275 acres involved. "Of that, 155 acres are in the golf course. The golf course provides a buffer between existing houses, and a visual buffer along Rte. 50 for those driving that busy road," Walsh said.

Because of the golf course, 57 percent of the site is open space.

Board Chairman Jack Herrity said he had had "some questions from Fairfax Farms residents about the impact on their well water."

But, "because of Difficult Run and the golf course, we do not think there will be any problems," Walsh said.

However, Herrity persisted, and Walsh asked to work on an agreement while debate continued.

The question of opening up nearby Valley Road to provide access for Fairfax Farms residents in and out of their neighborhood also was discussed, but the supervisors agreed that that was a separate issue that would have to be advertised and heard independently from the pending proposal.

One resident asked the county to "open up the east end of Valley Drive to Waples Mill Road," and produced 54 signatures supporting his request.

Other residents of Valley Drive said they opposed the idea.

"If we open it up, we won't be able to maintain our neighborhood and its rural character," said one Valley Drive resident.

Supervisor James Scott said a pending amendment to the county's comprehensive land-use plan for the area "does address the opening of Valley Drive," and told fellow supervisors there would be a forum for discussion of the proposal.

Residents testified on behalf of the Penderbrook proposal, including many of the original members of the 50/66 task force that developed the land-use plan for the Fair Oaks area.

Gretchen Davis, a Fairfax Farms resident, said, "This rezoning will buffer us from a high-density commercial development. This is a showcase example of residential development."

The plan for Penderbrook is "an anchor for residential development in the Fair Oaks area," Davis said.

Supervisor Martha Pennino said "residents have said the developer has gone beyond what the county requires."

If the golf course should cease operating, the land would be offered to the county park authority or held as common space within the development.