MIDDLEBORO — The state auditor’s office has agreed with Middleboro that the MBTA Communities Act fits the definition of an “unfunded mandated,’’ a decision welcomed by town officials who have been battling the state over the issue.

“This is an important victory for us,’’ said Select Board Chair Mark Germain, who has been a vocal critic of the state mandate.  

The act requires Massachusetts Bay Transit Authority, or MBTA, communities, including Middleboro, to allow multi-family housing developments of at least 15 units per acre by right in a zoning district within a half mile of the transit station. 

That could result in 1,471 additional units of housing in Middleboro, Germain said. 

Advocates say the measures addresses the housing shortage in the state and ensures access to mass transit. 

But Middleboro officials have argued that this requirement could cost the town $21 million in annual costs, including $16.5 million dollars in school expenses. These figures reflect the added number of students in the schools and the need for additional public safety personnel, among other expenses, town officials have said.

The town wrote, in a letter to the state auditor’s office, that the ruling constituted an unfunded mandate because the state provides no mechanism for covering costs associated with the zoning changes and their potential impact. 

This argument was supported by a ruling by the Division of Local Mandates, which is part of the state auditor’s office. 

Expenses resulting from any regulation that “imposes additional costs’’ beyond incidental fees should be paid for by the state, the opinion states. The town could also petition the court to be exempted from the mandate, the letter states. 

In a statement issued Feb. 24, the state Attorney General Andrea Joy Campbell disagreed with the auditor’s decision. 

“The auditor’s claim that the MBTA Communities Law is an unfunded mandate is wrong, and, more importantly, this letter has no effect whatsoever on implementation of the law,’’ her statement reads. “If those who oppose housing affordability try to make a similar claim in court, the state will vigorously defend the law, and we intend to be successful, as we have been so far.’’

This is the latest in a series of legal battles between the state and the town regarding the communities act. Middleboro filed a friend of the court brief last year in support of Milton’s refusal to submit a plan to the state to meet the required zoning changes.

The Supreme Judicial Court ruled in that case that the law is constitutional and the state can require zoning changes. The ruling also stated that the guidelines for the law were not effective and must be redone.

The state has ordered MBTA communities to submit their plans to comply with the act or potentially forfeit all grant money.

Special Town Meeting voters in October voted overwhelmingly to reject a potential plan, which means the town is not in compliance with the state mandate. 

That vote, Germain said at the Feb. 24 Select Board meeting, allowed the fight to continue. He thanked voters for their stance.

Still, he noted, “we haven’t won yet.’’ The town is still determining its next course of action, he said. 

Select Board member Brian Giovanoni, who took the lead on drafting the letter to the auditor, said he was “very happy’’ with the auditor’s decision.

“I love it,’’ he said. “Thank you to the state auditor.’’