Home/Pools & Additions
Building a Pool, Addition or Shed Near Your Septic Field
The question is rarely just where the drainfield is now. In Alabama it is also where the next one is supposed to go. Here are the distances, the reserve-field rule, and what to sort out before a contractor breaks ground.
How close can I build to my septic tank and drainfield?
Alabama's onsite rules set minimum horizontal distances from each part of the system. These are the ones that come up when someone plans a project, in feet, from rule 420-3-1-.65:
| From | To the drainfield (EDF) | To the tank, treatment unit, pump chamber or D-box |
|---|---|---|
| Building foundation | 5 | 5 |
| In-ground swimming pool | 10 | 5 |
| Basement or in-ground storm shelter | 15 | 5 |
| Basement or storm shelter with a drain | 25 | 5 |
| Property line | 5 | 5 |
| Potable water line | 5 | 5 |
| Embankment or cut | 25 | 5 |
| Drainage way, natural or man-made | 25 | 10 |
The full table, including wells and surface water, is on our setbacks page. The storm shelter rows matter more than people expect: a drained shelter has to sit five times as far from the field as a plain foundation.
The reserve field: the part of the lot you cannot use
Alabama requires a Replacement Effluent Disposal Field (REDF): a defined, documented area, separate from the working field, set aside in case the field ever has to be replaced. The rules then say two things that bite on a building project:
- A lot may not be built on or altered so that the EDF or the REDF is adversely affected.
- A lot may not be subdivided below its permitted size without a new permit application to the county health department.
So clearing the drainfield is not enough. A pool, addition or garage that lands on the reserve area is a problem even if the current field is nowhere near it.
Send us the address and what you want to build, and licensed local pros can locate the system and quote the work. Free, no obligation. Request free quotes →
Sinkholes and the 300 foot setback
St. Clair County sits in ridge and valley country with karst ground, which our karst page covers in detail. The rule that matters here: a drainfield must be 300 feet from caves, sinkholes and similar depressions, and a tank 50 feet. A geologist can reduce it with written documentation of a geological investigation. If a project means moving or replacing the field, that distance can decide where it is allowed to go.
Can an engineer reduce the setbacks?
For some of them. An engineer may reduce the setbacks for building foundations, drainage ways, embankments, interceptor drains and basements or storm shelters, with acceptable justification such as advanced treatment or drip disposal. That is a design decision for the county health department to accept, not something a contractor can assume.
Utility lines, driveways and new pipes
Projects bring trenches with them. Under the rules, no underground utility may cross over or under a drainfield pipe, and no system may sit in a utility easement for underground equipment. Where an effluent line crosses a water line, it needs 18 inches of vertical separation with the effluent line underneath; otherwise the water line is encased for 5 feet either side.
What to do before you build
Find the tank, the field and the reserve area
Ask the St. Clair County Health Department, which issues onsite permits here, what it holds on file for the property. If the layout is not documented, a licensed installer or evaluator can locate the system on the ground.
Mark the distances before the design is fixed
Measure from the table above with the pool or addition drawn in, including any drain from a basement or shelter.
Keep equipment off the field
Plan the access route for diggers and concrete trucks so it does not run over the field or the reserve area.
Questions people ask
How far does a swimming pool have to be from a septic drainfield in Alabama?
An in-ground pool must be at least 10 feet from the drainfield and 5 feet from the tank, under ADPH rule 420-3-1-.65.
Can I build an addition over the reserve septic field?
No. Alabama rules say a lot may not be built on or altered so that the drainfield or the replacement field is adversely affected.
How far must a storm shelter be from a septic field?
15 feet from the drainfield, or 25 feet if the shelter has a drain. Either way, 5 feet from the tank.
Can the setback distances be reduced?
Some can. An engineer may reduce the foundation, drainage way, embankment, interceptor drain and basement or storm shelter setbacks with acceptable justification, and a geologist may reduce the 300 foot sinkhole setback with a documented investigation.