N.H. Rev. Stat. Ann. § 540-B:10

The owner or agent of the owner of a shared facility may require a security deposit in an amount to be determined by the owner or agent of the owner. If the deposit amount is more than the equivalent of 30 days rent, there shall be a written instrument acknowledging receipt and explaining where the deposit shall be maintained and when it shall be returned. If there is no written agreement, the deposit shall be returned within 20 days after the occupant has vacated.