An agreement between Prince William County and a national developer would protect the county's oldest house and part of a Colonial-era highway in exchange for allowing an additional 300 condominiums, county officials said yesterday.

The county asked national developer D.R. Horton to remove 42 of 840 townhouses planned for land close to Rippon Lodge, a home built in 1745 that the county bought in 2000 for a museum. The Woodbridge development would have also sat astride one of the last existing segments of the historic highway U.S. and French troops used to reach the decisive Revolutionary War Battle of Yorktown.

County officials had hoped to persuade Horton to follow the lead of Centex Homes, which signed a widely hailed agreement with the county two years ago that preserved the Bristoe Station Civil War battlefield, just south of Manassas.

Horton responded this spring with a lawsuit that accused the county of illegally withholding permits for its already approved subdivision.

"We did not file suit lightly. I've never sued a county before," said Mike Jones, a Horton vice president. "But even if we won, everyone would be losers. We felt we had to find a way to turn this into a win-win. It was important to the county, and we thought it was the right thing to do."

The agreement provides for an additional buffer area between Rippon Lodge and the new Horton development. It would also preserve a part of the old Colonial highway that will be managed by the county's historic resources department. The additional 300 units are about 600 less than the maximum density allowed under county zoning.

County officials hailed the agreement as another step in saving what is left of the fast-developing county's history.

"Developers are finally catching on that we are serious about preserving our past," said Sean T. Connaughton (R), chairman of the county's Board of Supervisors.

County officials say Rippon Lodge is the oldest house in the county. It sits atop a tall ridge that commands a view of Neabsco Creek and the Potomac River, and comprises the last 40 acres of what was once a 21,000-acre tobacco and cotton plantation. County officials said the closest house under the original Horton plan would have been only 400 feet from the back of the Rippon Lodge house -- allowing visitors to the 18th-century home to take in views of 21st-century rear patios, air conditioners, swing sets and barbecue grills.

The townhouses also would have been built over a segment of King's Highway, which was the Interstate 95 of its day, linking all 13 original colonies. It is known as the Washington-Rochambeau route for its role in the run-up to the Battle of Yorktown.

The county bought the property in 2000 for $1.4 million from a descendant of the original owner. The county is in the midst of an expensive restoration of the house and grounds.

"Developers are finally catching on that we are serious about preserving our past," said Sean T. Connaughton, chairman of the county board.