Why Your Barndominium Isn’t a “Typical” House
Before we dive into pipes and permits, we need to address the elephant in the room: your barndominium is likely not a standard suburban home. These structures often feature large, open floor plans, but they also frequently include mixed-use spaces that a traditional septic designer might not anticipate.
You might have a dedicated workshop with a utility sink, a dog-washing station in the mudroom, or even a future guest suite planned for the loft. Perhaps you run a small business from home, or you anticipate heavy laundry use from farm-related activities . All of these factors change the “load” you put on your system.
Furthermore, because barndominiums are usually built on raw, undeveloped land, you don’t have the luxury of choosing a lot based on pre-existing utility hookups. You have to work with the dirt you have—which brings us to the very first, and most important, step.
Step 1: The Truth Lies Beneath – Soil and Perc Testing
You cannot design a septic system over the phone or by looking at a satellite image. The soil is the engine of your septic system. In fact, the soil does most of the actual “treatment” of the wastewater. If the soil fails, the system fails.
This is why a percolation test (perc test) is non-negotiable. A licensed professional will dig holes in your proposed drain field area, saturate the soil, and time how long it takes for the water to drain .
What the test tells you:
- Soil Composition: Is it sandy (drains too fast, risking groundwater contamination), clay (drains too slow, causing backups), or the golden child—loam (perfect balance)?
- Water Table Depth: If the seasonal high water table is too close to the surface, there isn’t enough “dry” soil to filter the waste before it hits the groundwater .
- Depth to Bedrock: You need several feet of workable soil. Rock prevents excavation for the drain field lines.
The Expert Tip: Never finalize your barndominium site plan or purchase the land contingent on a successful perc test. If the land fails a standard perc test, you aren’t necessarily out of luck—you just might need a more expensive engineered system—but you need to know that going in .
Step 2: Sizing the System – It’s About Bedrooms, Not Square Footage
One of the most common mistakes in barndominium design is undersizing the tank. Health departments generally use the number of bedrooms to calculate septic size because bedrooms indicate potential occupancy. However, your barndominium lifestyle might require an upgrade.
Here are the general guidelines for septic tank capacity :
- 1-2 bedrooms: 750–1,000 gallons
- 3 bedrooms: 1,000–1,250 gallons
- 4 bedrooms: 1,250–1,500 gallons
- 5+ bedrooms: 1,500–2,000+ gallons
When to Size Up:
If your barndominium includes any of the following, you should seriously consider moving to the next tank size or designing a system with a higher daily flow allowance:
- A workshop with a bathroom or slop sink.
- A home-based business (e.g., hair salon, small cafe).
- Plans for a future “mother-in-law” suite or guest quarters.
- Large soaking tubs or high-water-use fixtures .
It is far cheaper to install a slightly larger tank now than to dig one up and replace it in five years because you’re overloading it.
Step 3: Choosing the Right Type of System
Once you know how fast your soil drains and how much waste you need to treat, you can select the technology. There is no single “best” system—only the best system for your specific lot.
The Conventional Gravity System
This is the gold standard for simplicity and cost. If you have deep, well-draining soil and a gently sloping lot, gravity will do all the work. Waste flows from the house to the tank, and the liquid effluent (wastewater) flows by gravity to the drain field. It has no pumps, no electrical parts, and very little to break down .
The Pressure Distribution System
If your lot is flat (or has very little slope), you can’t rely on gravity to push the water through the drain field pipes evenly. A pressure distribution system uses a small pump in a dosing tank to push effluent out into the lines under pressure. This ensures the entire drain field is used evenly, which actually helps the soil stay healthy longer .
The Mound System
Is your worst nightmare a high water table or shallow bedrock? A mound system is your solution. Essentially, builders truck in a massive amount of sand to create an artificial drain field above the natural ground. The effluent is pumped up into this mound, treated in the imported sand, and then slowly filters down into the native soil. It’s effective but requires more space and maintenance, and it’s visible on your landscape .
The Aerobic Treatment Unit (ATU)
Think of this as a miniature municipal treatment plant. An ATU injects oxygen into the treatment process, which encourages aerobic bacteria to break down waste much more efficiently than the anaerobic bacteria in a standard tank. The result is incredibly clean effluent, which is great for environmentally sensitive areas. However, it requires electricity and more frequent maintenance checks .
Step 4: Site Planning – Where Does It All Go?
You know your soil type and your system type. Now, where do you put it? This is a puzzle that requires looking at a map of your property.
Critical Setback Distances:
You cannot just dig a hole anywhere. Local health departments enforce strict “setbacks” to protect drinking water and structural integrity. While these vary, here are common minimum distances :
- Well: 50–100 feet away (100 ft is a very common standard for drain fields).
- House Foundation: 10–20 feet away.
- Property Lines: 5–10 feet away.
- Surface Water (Streams/Ponds): 50+ feet away.
- Trees: Keep them away from the drain field; roots will destroy the lines.
The Barndominium Specifics:
Because you likely have a large concrete slab or apron, you must think about traffic. Do not route your septic system—especially the tank and distribution box—under an area where you plan to park a truck, RV, or heavy farm equipment. The weight compacts the soil and can crush pipes. Place the system in an area that is accessible for a pumper truck (they need to get within 50-100 feet of the tank) but out of the way of your daily work and living activities .
Step 5: The Permitting Maze
This is the part where you need to put on your patient hat. Septic permits are handled at the county or state level, and the rules are in place to protect public health.
You will generally need to submit the following to the local environmental health department:
- Perc test results.
- A detailed site plan showing the house, well (existing or proposed), septic tank, and drain field.
- System design calculations.
- Proof that a licensed installer will do the work .
Never, ever attempt to skip this process. A non-permitted system is a legal liability. When you go to sell your barndominium, the bank or buyer will require proof of a compliant system. If it’s not permitted, you could be forced to replace a perfectly good system just to meet code.
Step 6: Long-Term Stewardship
A septic system isn’t a “set it and forget it” appliance. If you take care of it, it can last 25 to 40 years .
- Pumping: You need to pump the tank every 3 to 5 years. This removes the non-decomposable “sludge” layer that builds up at the bottom. If you don’t pump it, solids will flow out into the drain field and clog the soil pores, destroying the field .
- Water Efficiency: The less water you send down the drain, the longer the effluent stays in the tank (letting solids settle) and the longer it rests in the soil. Fix leaky toilets and spread out laundry loads.
- Be Careful What You Flush: This is critical for the workshop area. Never pour harsh chemicals, paints, solvents, or excessive bleach down the drain. These kill the bacteria that make the system work . Workshop drains should ideally be separate from the residential septic if they will handle automotive fluids.
- Install Risers: When you install the tank, insist on having “risers” put in. These are concrete or plastic cylinders that bring the access lid from the top of the tank (3 feet underground) up to the surface. This makes pumping and inspections 100 times easier and cheaper .
The Bottom Line
Designing a septic system for your barndominium isn’t the most exciting part of the build, but it is arguably the most important for your long-term peace of mind. It is a system that marries the natural capacity of your land with the engineering of modern waste treatment.
Start early, test the soil before you finalize your plans, and work with local experts who understand the regulations in your area. By treating your wastewater system as an integral part of your rural dream home—rather than an annoying necessity—you ensure that your barndominium remains a sanctuary of comfort and safety for the long haul.