Carroll County home and business owners with more impervious surfaces may have to pay a larger share of the state-mandated tax on the rain coming off of people's driveways and roofs than others if the Carroll County Board of Commissioners approves a recommendation from a special advisory committee it set up.

The committee, which is made up of farmers, environmental advocates, water resource specialists and business owners, will meet Thursday to decide the best fee structure to charge a stormwater fee for residential homes and businesses in the county. The advisory committee will complete its final report and present it to the commissioners by the end of the month.

However, it will be up to the board of commissioners to decide the fee structure and the actual stormwater fee, according to Brenda Dinne, special projects coordinator for the county.

The commissioners have been outspoken opponents of the fee, also called the "Driveway Tax" and "Rain Tax."

"I cannot begin to project what the commissioners are going to decide," Dinne said.

In 2012, the Maryland General Assembly passed a law which requires Maryland's nine largest counties and Baltimore City to establish a watershed protection and restoration program that includes a stormwater remediation fee for impervious surfaces. The money collected by the new tax would be used to clean up the Chesapeake Bay.

The jurisdictions must adopt and implement the restoration program by July 1.

While the advisory committee has not officially decided on the fee structure, Dinne said it is leaning toward a plan.

The advisory committee supports implementing a graduated fee structure for residential homes based on the amount of impervious surfaces on the property, she said. Houses would be put into three different categories, each with a certain square footage of impervious surfaces.

The fee structure would work out so that residences with less impervious surfaces would pay a smaller fee and those with larger impervious surfaces would pay larger fees, Dinne said.

The advisory committee is also in favor of measuring the impervious surfaces of all businesses in the county. The fee to businesses would be dependent upon the amount of impervious surface at each location, Dinne said. The fee structure would be based on the amount of impervious surfaces at established square footage levels, she said.

"Our commercial and industrial base represent a smaller percent of all the properties in the county but represent a much greater percentage of impervious surfaces in the county," Dinne said.

The county already has an estimation of how much impervious surface is on each property as part of its National Pollutant Discharge Elimination System permit program, said Tom Devilbiss, deputy director of Carroll County's Department of Land Use, Planning and Development.

The NPDES permit program controls water pollution by regulating point sources that discharge pollutants into waters of the United States, according to the U.S. Environmental Protection Agency website.

Devilbiss said the county will not need to do a lot of additional work to figure out impervious surface amounts for each property.

The nine counties and Baltimore City have developed different ways to implement the stormwater fee, according to Les Knapp, legal and policy counsel for the Maryland Association of Counties. Frederick County passed a flat, 1-cent fee to property owners while Howard County has set a rate of $15 per 500-square feet of impervious surface.

"The flexibility is a good aspect of the bill," Knapp said. "While we opposed the overall legislation, MAC did advocate for making it as flexible as possible."