The On-Site Wastewater Treatment Facility Addendum is used when a property is served by a septic system or other on-site wastewater treatment system rather than a public sewer connection. It addresses the type of system, its condition, the location of the tank and leach field, permit status, and the buyer's right to have the system inspected during the due-diligence period.

Arizona requires that on-site wastewater systems be permitted and maintained in accordance with state and county regulations. The addendum typically requires the seller to have the system pumped and inspected by a licensed inspector before or during the inspection period, and to disclose any known defects or repairs.

For horse property and acreage buyers, septic system capacity is an important consideration. A system sized for a residential household may not be adequate if the property is used for guest quarters, employee housing, or other uses that increase wastewater volume. Understanding the system's capacity and condition before closing is essential.