Montana Title 67. Aeronautics § 67-7-209. Prior nonconforming uses

(1) All regulations adopted under this chapter must be reasonable and may not require the removal or alteration of any structure or tree or require cessation or alteration of a use that is lawfully in existence when the regulations become effective. Those structures, trees, or uses must be treated as prior nonconforming structures, trees, or uses that may remain or continue, but regulations may prohibit their expansion or their reconstruction or replacement following destruction or substantial damage. For the purposes of this section, “substantial damage” has occurred when 80% or more of a structure or tree is deteriorated or decayed or has been torn down or destroyed.

(2) The regulations may require that trees in place at the time that the regulations take effect be maintained by the political subdivision, at its expense, at heights attained at that time.

(3) The regulations may require the owner of structures or trees to permit the political subdivision, at its expense, to install, operate, and maintain the lights and markers necessary to warn pilots of the presence of an airport hazard.

(4) Land in existing residential subdivisions or platted for residential subdivision at the time that regulations are adopted may continue to be used for residential purposes, subject to notification provided to property owners that the lots are within an adopted airport affected area.