Mitchell Rales speaks at a ceremony on his property, Glenstone, in Potomac, Md., on June 24, 2013. Behind Rales is a sculpture created by artist Jeff Koons that contains 27,000 plants. (Bonnie Jo Mount/The Washington Post)

The foundation that operates Potomac billionaire Mitchell Rales’s private art museum is getting a $372,993 tax refund from Montgomery County.

The refund reflects a change approved by the County Council last week after officials decided it was unfair to tax a building that will house much of Rales’s art collection at the same rate as some commercial establishments.

“We have to ask ourselves what we want in this county,” council member Hans Riemer (D-At Large) asked just before the vote. “Does the county want to levy heavy taxes on a museum as though it were the same thing as housing or commercial construction? This is a social benefit we don’t want to create barriers to.”

The unanimous passage of Bill 62-14 — “Development Impact Taxes — Exemptions — Ancillary Facilities” — reduced the tax due on the new exhibition space from $445,419 to $72,426.

The change illustrates the challenges inherent in deciding how much those who want to build in Montgomery County should pay to help address the impact of new construction on traffic and roads. And the quiet manner in which the change was made — without a separate public hearing or debate — drew criticism from one council member and a key member of the council’s staff.

Rales, through a spokesman, did not respond to multiple interview requests.

Montgomery’s transportation impact tax is a one-time payment assessed on new projects to help pay the costs of increased traffic on congested roads. The county collected about $14 million in impact taxes in the last fiscal year.

The tax rate varies. An office building in most parts of the county would be assessed at $12.30 per square foot, or $1.23 million for 100,000 square feet of space. Other buildings — such as houses of worship — pay much less because of their value to the larger community. Hospitals, ­social service agencies and ­affordable housing units are ­exempt.

But county officials had seen nothing quite like the 150,000-square-foot addition Rales is building at Glenstone, the white modernist building on his Potomac estate that houses one of the world’s leading private collections of modern art.

When the project was announced, it was described as comparable in size and scope to the National Gallery of Art’s East Building. It will allow Rales and his wife to permanently exhibit much more of their extensive collection, which includes works by Alexander Calder, Henri Matisse and Mark Rothko.

There was no specific “museum” category in the county’s tax charts. So officials placed Glenstone under “Other Non-Residential,” a grab-bag of land uses that includes funeral parlors, dog kennels and sewage treatment plants.

They levied a tax of $6.15 per square foot for the new building. The Glenstone Foundation appealed the $445,419 tax bill in October, arguing that the museum should be categorized as a “social service provider” — exempting it from the impact tax — because it hosted small groups of Montgomery County Public School students during the school year. The appeal was denied, and the foundation paid the tax bill late last year.

That’s where the story might have ended. But Diane Schwartz Jones, director of the county Department of Permitting Services, which collects impact taxes, thought Glenstone’s payment seemed “hefty,” especially for a nonprofit entity that would benefit county school children and would not draw heavy traffic.

She suggested that County Executive Isiah Leggett (D) create an impact tax category — “cultural institution” — paralleling a category that had been included in the county’s recently rewritten zoning code. She set the rate at $1 per square foot, the same as for private schools. It cut Glenstone’s impact payment by more than 80 percent.

The council approved the change as an amendment to another bill related to the impact tax — this one eliminating the levy for ancillary buildings at existing institutions if they are not expected to draw additional traffic.

Because the “cultural institution” category was not introduced as a separate bill, it did not receive its own public hearing, a situation that triggered concern from Michael Faden, then the council’s senior legislative attorney. “It’s a new concept, and it hasn’t had a public vetting at all,” Faden said at a Government ­Operations Committee hearing Jan. 29.

In a separate memo to council members, Faden — who was days away from his Feb. 1 retirement — said such changes in tax policy are “appropriate only when those changes are urgently needed, and we have not been given any reason why that is the case here.”

Nancy Navarro (D-Midcounty), chair of the Government Operations Committee, also favored a separate hearing “to examine the cost and what other types of categories this would affect.”

But she was voted down by committee members Rie­mer and Sidney Katz (D-Rockville-Gaithersburg). Riemer said the amendment was consistent with the intent of the ancillary buildings bill and did not require its own airing.

Some activists said they were disappointed by the lack of transparency, which they attributed to Rales’s wealth and influence. Rales won a controversial battle to install a sewer line at Glenstone several years ago.

State records show that Rales and his family and corporations donated a total of more than $40,000 during the 2014 election cycle to Leggett, Riemer, council member Roger Berliner (D-Potomac-Bethesda) and others.

“They give him whatever he wants,” said Ginny Barnes of the Western Montgomery County Citizens Association. “But my chief concern is the lack of transparency in granting this exception to the transportation impact tax. We knew nothing about this issue, and we are not hiding under a rock.”

Asked whether the donations might have influenced the county’s action, Leggett scoffed. He called the $24,000 he had received from the Rales family “a very small pittance” compared with the nearly $1 million he spent in his election campaign.

“They’re wealthy people, but that’s not the standard,” Leggett said Thursday. Glenstone, he said, “is clearly a very valuable asset to the county.”