A Plumbing Permit is required for all changes to plumbing, including installation or replacement of a water softener, water heater, or back flow prevention system for lawn irrigation. For water heater replacements, a fresh air intake must be installed if not existing.
Plumbers must have a current state license and bond through the Minnesota Department of Labor and Industry. You can check the status on their website - MN Dept of Labor & Industry - Contractor Lookup