Contest of mayors - upgrading zones/buildings not completing assignments

Go to the zone you want to upgrade.  Create a new house specific for that zone but don't upgrade the house yet.  When there is no room for a new house, bulldoze a house and rebuild one specific to that zone. Then go to the assignment list and select the assignment to upgrade that zone.  Upgrade the house, and the assignment is completed.    Collect your assignment reward and have fun!