A reserve area is land set aside on a property’s septic permit for a future replacement drainfield, kept undeveloped even though the current field is the one actually in use. Most state codes require it precisely because drainfields eventually fail, and having qualifying soil already identified and protected means a failure can be fixed by building a new field instead of only being able to repair the old one in place.
The reserve area has to meet the same soil and site requirements as the active field, since it needs to be able to actually support a replacement system when the time comes, not just be spare land on the lot. That is also why it is identified during the original soil evaluation rather than chosen later, when suitable soil elsewhere on the lot might no longer be available.
Building on, paving, or otherwise disturbing the reserve area defeats its purpose even though it is not actively part of the system yet. A pool, addition, or driveway placed there can permanently remove the option of an in-kind replacement, which becomes a real problem exactly when the active field eventually fails.
The property’s permit file or as-built drawing typically shows both the active field and the reserve area, which is worth checking before planning any new construction on a septic property, not just before building directly over the visible field.
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No, it is land held aside for a future replacement drainfield, not part of the active system, but it must still stay undeveloped.
Because drainfields eventually fail, and having pre-identified suitable soil protected on the lot means a failure can be fixed with a proper new field rather than being limited to repairing the old one in place.
Doing so risks losing the ability to build an in-kind replacement field later, right when the active field eventually needs one, so it is generally not allowed under the permit.
Guide last reviewed 2026-08-31.