Wis. Stat. § 145.195: Building on unsewered property.

(1) No county, city, town or village may issue a building permit for construction of any structure requiring connection to a private on-site wastewater treatment system unless a private on-site wastewater treatment system satisfying all applicable regulations already exists to serve the proposed structure or all permits necessary to install a private on-site wastewater treatment system have been obtained.

(2) Before issuing a building permit for construction of any structure on property not served by a municipal sewage treatment plant, the county, city, town or village shall determine that the proposed construction does not interfere with a functioning private on-site wastewater treatment system. The county, city, town or village may require building permit applicants to submit a detailed plan of the owner’s existing private on-site wastewater treatment system.