NASHVILLE (AP) — A free-market think tank is suing Nashville's government over a new policy to expand affordable housing.

The Beacon Center of Tennessee, alongside Southeastern Legal Foundation, filed Monday's lawsuit in Davidson County Chancery Court.

According to The Tennessean (http://tnne.ws/2oYtv9c), the lawsuit challenges the policy requiring apartment developers building five or more units to include a percentage of new affordable or workforce units if they request certain zoning variances.

The ordinance takes effect in June. Nashville will offer grants to offset added developer costs.

The lawsuit contends the policy violates a state law prohibiting municipalities from requiring affordable housing in new residential development.

Ordinance supporters contend it's voluntary because it's triggered when developers choose to apply for new development rights.

Metro Department of Law Director Jon Cooper declined comment on pending litigation.