Frequently Asked Questions How do I know how large my tank is? If your seller or builder did not leave you an “as built” plan, you may request one from you health department (you will need your lot and block number). An “as-built” plan is provided by the installer, which contains actual dimensions to key septic components. CJS has an extensive file of “as-built” septic plans; you may want to check with us first. Return to FAQ's