Percolation Testing: (AKA: Perc Test, Perk Test, Septic Test) Percolation tests are required to determine the feasibility of installing an Onsite Wastewater Treatment System (septic system). Septic system design is part of a perc test. Perk tests are usually required on lots for sale and in escrow which will require a septic system, the close of escrow is contingent upon a passing percolation test. Perc testing is often required when building room additions or second units on lots with septic. Perc tests are beneficial because they provide a way to ensure that your septic system is large enough to meet the demand coming from the home and the properties of the soil.