It is usually not a good idea to build a deck near or on top of a septic tank. Most zoning ordinances will require that you maintain at least a 5' setback from an underground septic system. Installing frost footings and applying deck loads over a septic tank could result in damaging the tank or waste lines. A punctured septic tank will create a terrible mess and an expensive problem. Septic tanks also require access for maintenance. Abandoned or out of use septic tanks should also be avoided. Some older properties have buried septic systems that are not marked on surveys along with other utilities and easements. This can be an unpleasant surprise when installing footings for a deck.