N.H. Rev. Stat. Ann. § 524:2

Any person against whom an action is brought for the recovery of real estate may, with the plea, file a brief statement, setting forth that such person and the persons under whom such person makes the claim have been in the actual peaceable possession of such real estate, under a supposed legal title, for more than 6 years before the action was begun, and that the value of the real estate has been increased by them by buildings or other improvements.