Rent charged for residential estates may not be increased if the dwelling unit is in violation of the warranty of habitability. Any violation caused by the tenant, his family, guests or invitees shall not bar a rent increase. A written or oral waiver of this requirement is against public policy and is void. Any person in violation of this section shall be liable for the return of any sums unlawfully obtained from the lessee, with interest and reasonable attorneys' fees and costs.   [PL 1985, c. 293, §5 (AMD).]