No structure may be built on top of any portion of a septic system.  A setback of 10 feet is required between any structure and the septic tank, and 20 feet between any structure and the soil absorption system.  In addition, no structure may be built within the space reserved for absorption system replacement, or within the 20 foot setback distance of that area, in accordance with 22 GAR §12105(d).
There are numerous reasons for this, based on both engineering and common sense.  The regulations require that the septic tank and leaching field must remain accessible for maintenance and repair, as necessary, and that no structures be built which would prevent the use of the 100% reserve absorption system in the event of a failure.  Other, more technical reasons include precautions to prevent damage to your home or dangers to life and health.  Building too close to a leaching field may cause your foundation to settle and damage your home.  Building too close to or on top of a leaching field or septic tank is also likely to result in the introduction of odors and gases to indoor spaces, which can be both hazardous to life and health, and create a risk of explosion or fire.