Mo. Rev. Stat. § 569.050: Arson in the second degree — penalty.

1. A person commits the offense of arson in the second degree if he or she knowingly damages a building or inhabitable structure by starting a fire or causing an explosion.

2. A person does not commit an offense under this section if:

(1) No person other than himself or herself has a possessory, proprietary or security interest in the damaged building, or if other persons have those interests, all of them consented to his or her conduct; and

(2) The person's sole purpose was to destroy or damage the building for a lawful and proper purpose.

3. The defendant shall have the burden of injecting the issue under subsection 2 of this section.

4. The offense of arson in the second degree is a class D felony unless a person has suffered serious physical injury or has died as a result of the fire or explosion, in which case it is a class B felony.