We hold that the language of RSA 540:13-d is unambiguous. According to the plain language of the statute, when a premises leased or *85rented for residential purposes is in substantial violation of the standards of fitness for health and safety, and the violation materially affects the habitability of the premises, the tenant has an affirmative defense to an action for possession based on non-payment of rent. See Penrich, Inc. v. Sullivan, 140 N.H. 583, 588-89 (1995). This defense precludes the landlord from maintaining an action for non-payment of rent so long as the four factors set forth in the statute have been satisfied. See RSA 540:13-d, I. In this case, the trial court specifically found that the apartment was in a dilapidated condition but ruled that the defendants could not raise the RSA 540:13-d defense because the apartment was in need of substantial rehabilitation, rather than mere repair. The statute does not distinguish between repairs and more substantial rehabilitation. The plain language of the statute recognizes that the affirmative defense applies in situations where there is a “substantial” violation of the standards of fitness for health and safety that “materially” affects the habitability of the premises. We will not consider what the legislature might have said nor add words that it did not see fit to include. Remington, 150 N.H. at 654. Because we conclude that the legislature intended to make the RSA 540:13-d defense available in all instances where there is a substantial violation of the standards of fitness for health and safety that materially affects the habitability of the premises, we hold that the trial court erred when it ruled that the defense was not available to the defendants. Reversed and remanded. Broderick, Nadeau, Dalianis and Galway, JJ., concurred.