Builders and local officials are mounting a campaign to delay or change new state rules aimed at reducing storm-water pollution, arguing that they will undermine Maryland's Smart Growth policy by discouraging redevelopment in cities, towns and older suburbs.

The rules, finalized last year, carry out a 2007 law that requires developers across the state to do much more than before to keep rainfall from washing pollutants off lawns, roofs and pavement in their projects. Such storm runoff is a significant and growing source of the fertilizer and other contaminants that are fouling local streams and rivers and the Chesapeake Bay, officials say. Environmental advocates have hailed the law as a major new tool for helping to restore the bay.

But complaints about the new rules, which take effect May 4, have grown to the point that lawmakers and aides to Gov. Martin O'Malley joined a standing-room crowd Friday at the Maryland Department of the Environment to hear a parade of developers, mayors and planners warn that the mandates could encourage sprawl. Several builders said the changes they'd need to make to long-planned projects would add hundreds of thousands and even millions of dollars to the cost of their projects.

"For redevelopment, it's going to be so expensive that it's a whole lot better to go [build in] a cornfield somewhere," said Michael C. Powell, a lawyer who represents the Maryland State Builders Association.

Powell and others called on state officials to "grandfather" or exempt from the new requirements those housing and commercial projects already in the development pipeline. It's unclear how many there are, though estimates run into the thousands or even tens of thousands. They also insisted that the rules need to be more "flexible" regarding redevelopment projects.

The law aims to ensure that runoff from developed land be no different than it was before construction started. Developers are required to follow what's called "environmental site design," cutting down on the amount of pavement and allowing rainfall to soak into the ground instead of collecting it in drains and piping it into large ponds or nearby streams.

Under the regulations, redevelopments do not have to control and treat as much storm water as do new developments. But builders complain the rules have a narrow definition of redevelopment, forcing them on many projects in built-up urban and suburban areas to meet the tougher standards for building on virgin land.

Local officials said they worried that the regulations could choke off redevelopment, which they say is essential to revitalizing aging neighborhoods and stagnant communities.

"In my opinion, no growth is not Smart Growth," said Peter C. Fosselman, mayor of Kensington in Montgomery County. He and other local officials also complain that the rules would force them to rewrite local laws and do more reviews and inspections when they're struggling to balance their budgets.

Environmental advocates defended the regulations, pointing out that developers are dealing with similar or even tougher requirements in some cities and counties, including Philadelphia. Local governments are facing increased state and federal pressure to help revive the lagging cleanup of the Chesapeake, activists noted, with development accounting for about one-fourth of the pollution fouling the water.

"The bay is in critical care, and we must act with great boldness to restore the Chesapeake Bay," said Diane M. Cameron of the Natural Resources Defense Council.

Robert M. Summers, deputy secretary of the environment, defended the regulations, saying they'd been written to give developers and local officials flexibility in how to achieve the needed controls on runoff. But he acknowledged the outpouring of complaints and said department officials are reviewing the rules to see if changes are in order.

"We want to encourage redevelopment," he said, "but at the same time we need to develop better - and significantly better - when we do redevelop, or we won't make progress on cleaning up the bay or cleaning up our waterways."

Critics of the new rules are preparing to seek legislative relief.

Del. Maggie McIntosh, chair of the House Environmental Matters Committee, sat in on the forum, as did several other legislators. Despite assurances from environmental officials that they'll consider adjusting the regulations in response to the complaints, the Baltimore Democrat said she expected the General Assembly would move to ensure that the storm-water measure doesn't cause unintended consequences.

"We do not want to make it prohibitive or too costly to do urban or infill redevelopment," she said. "That will not provide smart growth. It will do just the opposite."