Fairfax County residents looking to build sunrooms, patios and other home additions may have to endure a flood of new regulations that are intended to protect the Chesapeake Bay but could double the cost of construction, officials said.

Virginia law now requires each community to control its flow of excess water from rain storms or snow melts. The rules are intended to limit the amount of pollutants the water can pick up from surfaces like parking lots and carry into the bay, lawmakers said.

But even as the state toughens its stormwater rules, Fairfax County officials decided the state isn't going far enough and ordered their own, tougher restrictions. The proposed Fairfax rules, however, are so strict that some officials said they could prevent county homeowners from putting any kind of addition on to their houses -- or force them to pay and install other amenities, like a rain garden, to offset the effects of the new addition on ground water.

"As they have it designed, even a single addition becomes prohibited," said Supervisor John Cook, R-Braddock. "It's one thing if you're building a basement, but the person who wants to go out there and tack on a sunroom couldn't do it."

A preliminary version of the rules drafted by county staff would restrict homeowners to covering no more than 10 percent of their lots with concrete or other impervious materials. If the impervious surface created by the addition pushes the house beyond the 10 percent benchmark, owners would have to install a rain garden or other mechanism to help clear impurities from the water before it reaches the Bay.

The Fairfax law goes beyond the state regulations, which apply only to lots of one acre or more. The state law also doesn't limit how much of a lot can be covered by a house or other impervious materials.

"I think we're going above and beyond what the state's requiring because of a couple of bad apples," said Supervisor Pat Herrity, R-Springfield. "There was an occasional issue [with residents], but this decision is going to impact a whole host of people. Because of that, we need to change our ordinance to make it more reasonable."

Fairfax County has until December to adopt the plan and doesn't need to implement it until July 2014, giving county staff time to adjust their recommendations.

"We have to maintain a position where we're encouraging home improvement," said Supervisor Jeff McKay, D-Lee. "There's a difference between building a whole new house and adding something on."

Homeowners who seek to gut and then rebuild on their existing lots also would be affected by proposed changes, Cook said, because they too would be responsible for building amenities to capture water should their project pass the 10 percent mark.

Although the impact that the new ordinance would have on existing homes was not laid out in the preliminary report, it's the residents who simply want to add on to their homes that would be hit the hardest, he added, because they could see the price of their project double because of required add-ons to minimize water flow.

"I think there's still some tweaking to be done," said Supervisor Penny Gross, D-Mason. "We knew this was going to be hard."