The Montgomery County Council on Tuesday unanimously approved a sewer line for billionaire businessman Mitchell Rales’ Glenstone art gallery.

The proposal now goes to the Maryland Department of the Environment for further review. Last week, the council signaled that it would back the sewer despite its rejection by the county planning board

Glenstone, off Glen Road in Potomac, is in a part of the county where septic systems are required as a means of controlling growth. But Rales, whose art collection is widely considered one of the most extensive post-World War II assemblages of abstract expressionists, had sought a dispensation from the ban.

The gallery is open to the public Thursdays and Fridays by appointment only. Rales has said he wants to build a new, 125,000-square-foot gallery that would rival the East Building of the National Gallery of Art. To do so, he said, a sewer connection would be preferable, because septic systems have a life span of 25 to 50 years.

The Montgomery County Council agreed, and Tuesday granted Rales’s request. The sewer will be limited to the buildings housing the art collection and will not be connected to Rales’s house, also on the 200-acre property.

Should he sell the property, which he says he does not plan to do, the new owner would have to seek county permission for any further sewer connections.

“We are grateful that the county council has unanimously approved public sewer for Glenstone. We also want to thank the many people who have supported our effort to expand Glenstone in the most thoughtful and environmentally responsible way,” said Charles Maier, a Rales spokesman.