Short answer: Generally no. Drainfields need unsaturated soil, oxygen exchange, access, and protection from compaction. Buildings, patios, pools, and other improvements can interfere with treatment and future repair.
Drainfields are treatment areas
The drainfield is not simply empty yard space. It distributes septic effluent into soil where further treatment occurs.
Reserve area
Many jurisdictions also protect a replacement or reserve area. That space can be just as important as the currently active field when planning additions.
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What to verify locally
Permitting, inspection, setbacks, acceptable system types, professional licensing, and design requirements are controlled by state and local authorities. If your question affects construction, a property purchase, a repair, or drinking-water safety, confirm the answer with the local health/environmental department.
Primary reference
U.S. EPA — Frequent Questions on Septic Systems ↗
U.S. EPA — How to Care for Your Septic System ↗
Reviewed August 26, 2026.