To control the quantity and quality of storm water runoff
Provide minimum standards for site grading and control of storm water
For any new buildings where the grade of the lot is altered more than three feet. The exception is for new single family residences where the original slope is less than 5%. These permits are governed by Missoula Municipal Code 13.27 (PDF).
Permits expire if work is not started within 180 days of issuance.