Check with Thurston County Building Development Center, 360-786-5490, for setbacks and other requirements for septic tanks and drainfields. Determine if there is enough room for the septic system, replacement area, and drinking water well, if needed. Determine the soil characteristics, as the amount of sand, silt, and clay in the soil has an impact on the drainfield. Because there are many other considerations, including slope of the land, potential for flooding, in a wetlands area, nearness to water bodies, etc., seeking the help of a professional may be necessary.