N.H. Rev. Stat. Ann. § 154:20

If any building is deemed by the firewards or engineers to be dangerous to the property of others, by reason of decay, want of repair or otherwise, they may give written notice to the owner, the same to be served personally or by registered mail, to repair or alter the same, which shall contain a particular account of the repairs or alterations required to be made. If the owner does not reside in the town like notice may be served on the occupant, if any.