Charles County commissioners on Tuesday opened the Waldorf area to intensive city-style development, approving rules that will allow taller apartment buildings and densely populated neighborhoods that mix shops and homes.

Developers may use the new rules only for projects that encompass 100 acres or more, and only in Waldorf's designated urban core. Projects would need approval from the elected Board of Commissioners and from the Planning Commission that it appoints.

The new rules received unanimous voice-vote approval with all five commissioners present. Their vote came 14 months after they first took up the topic, and 11 months after commissioners decided to postpone action beyond last fall's election campaign, during which the pace and extent of development was a major issue.

"We think this is a smart choice for Charles County," said Board of Commissioners President Murray D. Levy (D-At Large). "We're comfortable with the adoption . . . because it only allows us to do something--it doesn't mandate. We can still say no."

The new rules will allow as many as 27 dwelling units on an acre, compared with the average of 10 housing units per acre permitted in the typical Charles County town house development. Developers could build seven-story apartment houses, compared with the normal five-story maximum.

Critics worry that such densities could burden schools and roads, and possibly detract from nearby property values.

Proponents say the rules will allow developers to build upscale districts that will produce jobs and attract higher-income residents.

The rules allow a mix of uses so developers can build pedestrian-friendly districts that combine shops, restaurants, homes and offices in a way that could recall old-fashioned, small-town downtowns.

This is a sharp contrast with the traditional suburban zoning that has guided most local construction, keeping businesses separated from homes and strictly controlling housing density.

Partly because of the density allowed under the new rules, planners expect the new districts will generate demand for mass transit, by bus at first and later by light rail links.

The rules tell county planning officials to favor projects that are near the railway that runs north and south through the middle of Waldorf, and that are closer to a major highway, such as U.S. Route 301.

To take full advantage of the greater housing densities allowed under the new rules, developers will be required to purchase development rights from county farms. Commissioners said that would bolster efforts to preserve farmland.

David W. Cooksey, the private land use consultant who proposed the rules change and ushered it through nearly two years of county hearings, said Tuesday's vote would usher in higher-quality development.

Cooksey, a former county planning chief, said he likely would bring a project forward under the new rules early next year. He declined to identify the client involved.

In March, officials with the Waldorf-based Chaney group of companies made public a proposal that included mixed retail and residential neighborhoods on 600 acres in Waldorf. Chaney president Francis H. Chaney II later wrote to commissioners, saying the rules change would help those plans.

Bob Agee, a Chaney executive, said he was pleased with Tuesday's vote.

"It gives us the opportunity to be a little creative," Agee said.