So you want to add a guest house out back. Or convert your shop to an apartment. Or build that ADU everyone’s talking about for rental income or aging parents. Great idea, but… have you thought about your septic system?
“It’s just one more bathroom, how much difference can it make?”
A lot, actually. And Idaho health districts take this seriously. Adding plumbing to accessory structures without proper septic capacity isn’t just against code - it’s a fast track to system failure, expensive fines, and potentially being forced to connect to municipal sewer (if available) or upgrade your entire septic system.
Let me walk you through everything about accessory use and septic systems in Idaho. Because that ADU is awesome until it kills your drainfield.
What Counts as Accessory Use?
Accessory use is anything with plumbing that’s not your primary residence. Common examples:
Accessory Dwelling Units (ADUs)
- Separate living space with full kitchen and bathroom
- Above garage apartment
- Detached guest house
- Basement apartment with separate entrance
- Converted garage or outbuilding
Shops and Outbuildings with Plumbing
- Woodworking shop with bathroom
- Auto shop with utility sink and toilet
- Barn with plumbing for livestock or human use
- She-shed/man-cave with bathroom
- Pool house with changing room and toilet
RV Connections
- Permanent RV hookup pad
- Sewer connection for recreational vehicle
- “Parking spot with utilities” that becomes semi-permanent
Guest Houses
- Separate building for visitors
- Mother-in-law quarters
- Caretaker residence on property
If it generates wastewater and connects to your septic, it’s accessory use requiring evaluation.
When You Need an Accessory Use Permit
Idaho health districts require permits for adding plumbing capacity to your property. You need an accessory use application when:
Adding New Plumbing
Didn’t have plumbing before, adding it now. Your shop had no bathroom, you want to add one. That’s new capacity requiring evaluation.
Even if you’re not adding a bedroom - just a toilet and sink - it still generates wastewater that your septic must handle.
Converting Space to Living Area
That storage room above the garage is becoming a studio apartment. Or the shop is getting converted to guest quarters. Any conversion to living space with plumbing needs accessory use permit.
Increasing Bedroom Count
This is the big one. Septic systems are sized by bedrooms. Add a bedroom to main house? Accessory use evaluation. Building ADU with 2 bedrooms? Accessory use permit.
The calculation: Original house bedrooms + ADU bedrooms = total bedrooms system must support.
Adding RV Connection
Permanent RV pad with sewer hookup counts as accessory use. Why? Because that RV might generate wastewater year-round if someone lives in it.
Temporary guest parking is different than permanent connection. Health districts distinguish between occasional use and regular/permanent capacity.
Why Septic Capacity Matters
Your septic system was designed for specific wastewater volume based on original bedroom count. Standard calculation:
3-bedroom house generates: 450-600 gallons per day 4-bedroom house generates: 600-800 gallons per day Each additional bedroom adds: 150-200 gallons per day
Your drainfield was sized for those volumes. Add more capacity without upgrading the system and you’re overloading it. Results:
- Hydraulic failure (flooding)
- Premature system failure
- Contaminated groundwater
- Property damage
- Health hazards
A drainfield sized for 3 bedrooms (800 sq ft typically) might need 1,200 sq ft for 5 bedrooms. That’s 50% more capacity. Can’t just squeeze it through existing system.
The Application Process
Here’s what Central District Health (and most Idaho districts) requires:
Step 1: Gather Information
You’ll need:
- Property legal description (from your deed)
- Year existing septic was installed (hopefully you have as-built)
- Year home was built
- Current number of bedrooms in main house
- Number of bedrooms/bathrooms adding
- Plot plan showing existing structures, septic system, wells, and proposed addition
- Building plans for the addition
If you don’t have septic records, health district might have them. If not, you’ll need to have your tank sized or pumped and provide receipt proving capacity.
Step 2: Create Detailed Plot Plan
Your plot plan must show:
- All existing buildings (house, shop, garage, etc.)
- Location of existing septic tank
- Location of existing drainfield
- Location of reserve drainfield area (hopefully you have one)
- Well locations (yours and neighbors within 100 feet)
- Water lines
- Property lines with setback distances
- Ditches, streams, and surface water
- Proposed addition location with dimensions
Doesn’t need to be professional survey quality, but it needs to be accurate and complete. Hand-drawn is fine if clear.
Step 3: Provide Narrative Description
Explain what you’re doing:
- “Converting 400 sq ft of existing shop to 1-bedroom ADU with bathroom and kitchenette”
- “Adding bathroom to existing barn for farm use”
- “Building detached 800 sq ft guest house with 2 bedrooms and bathroom”
Be specific. Vague descriptions get sent back for clarification.
Step 4: Submit Application and Fee
Take everything to health district office. Fees vary:
- Central District Health: $200-$400 typically
- Other districts: $150-$350
Non-refundable once they start evaluation, so make sure your application is complete.
Step 5: Inspector Evaluation
Health district reviews:
- Current system capacity
- Proposed additional load
- Adequacy of existing drainfield
- Availability of reserve area
- Soil conditions if new drainfield needed
- Separation distances for any expansion
Might require site visit to verify existing system and evaluate reserve area.
Step 6: Approval, Conditional Approval, or Denial
Approved: Existing system can handle additional load. You’re good to proceed.
Approved with Conditions: Might work if you do X, Y, Z. Common conditions:
- Reduce bedroom count
- Install water-saving fixtures
- Pump tank more frequently
- Monitor system performance
- Add reserve drainfield area if future expansion planned
Denied: Current system can’t handle additional load. Must either:
- Upgrade septic system before adding accessory use
- Reduce scope of addition
- Don’t add the plumbing
Common Scenarios and Solutions
Let me walk through typical situations:
Scenario 1: ADU on Property with Adequate Reserve
Situation: 3-bedroom house with 1,000 sq ft drainfield, 1,000 sq ft reserve area available. Want to add 1-bedroom ADU.
Analysis: 4 bedrooms total needs approximately 1,200 sq ft. Existing 1,000 sq ft plus 200 sq ft from reserve = enough capacity.
Solution: Install additional 200 sq ft drainfield in reserve area. Cost: $3,000-$6,000.
Permit: Approved with condition to expand drainfield before occupying ADU.
Scenario 2: Shop Bathroom on Maxed-Out System
Situation: 4-bedroom house with 1,200 sq ft drainfield, no reserve area, all usable property already committed. Want to add bathroom to shop.
Analysis: Even one bathroom adds 150 gallons per day. System already at capacity.
Solution Options:
- Install separate small septic system just for shop ($8,000-$15,000)
- Reduce main house to 3-bedroom (if feasible) to free capacity
- Connect to municipal sewer if available ($10,000-$30,000 for line extension)
- Don’t add the bathroom
Permit: Likely denied unless you pursue one of these alternatives.
Scenario 3: Guest House on Rural Property
Situation: 3-bedroom house on 5 acres, great soil, plenty of space. Want 2-bedroom guest house.
Analysis: 5 bedrooms total needs 1,500+ sq ft drainfield. Have space and good soil.
Solution: Expand drainfield to handle 5 bedrooms, or install separate system for guest house.
Permit: Approved after expanding capacity. Might be easier to do separate system if guest house is far from main house.
Scenario 4: RV Pad for Aging Parents
Situation: Want to park parents’ RV on property long-term with sewer hookup.
Analysis: RV counts as accessory use if occupied regularly. Generates wastewater like 1-2 bedroom unit depending on RV size.
Solution: Evaluate capacity for additional 2 bedrooms equivalent. Might need system expansion.
Permit: Required if RV will be occupied more than temporarily. Temporary guest use (few weeks per year) typically doesn’t require accessory use permit.
Scenario 5: Basement Apartment Rental
Situation: Finish basement with bedroom, bathroom, kitchenette for rental income. House currently 3 bedrooms with basement unfinished.
Analysis: Going from 3 bedrooms to 4 bedrooms. Need additional 200 sq ft drainfield capacity minimum.
Solution: If reserve area exists, expand into it. If not, might need complete drainfield replacement or system upgrade.
Permit: Required. Common scenario in Boise area as ADUs gain popularity. Health district will check thoroughly.
Capacity Calculations: The Details
Health districts use specific formulas:
Daily Flow Calculations
2-bedroom: 300 gallons per day minimum 3-bedroom: 450 gallons per day 4-bedroom: 600 gallons per day 5-bedroom: 750 gallons per day Each additional bedroom: +150 gallons per day
These are minimums. Actual use might be higher, but design is based on these standards.
Drainfield Sizing
Based on soil type and percolation rates. Typical Idaho soils:
Good soil (sandy loam):
- 2-bedroom: 600 sq ft
- 3-bedroom: 800 sq ft
- 4-bedroom: 1,000 sq ft
- 5-bedroom: 1,250 sq ft
Moderate soil (silt loam):
- 2-bedroom: 800 sq ft
- 3-bedroom: 1,100 sq ft
- 4-bedroom: 1,400 sq ft
- 5-bedroom: 1,700 sq ft
Poor soil (clay):
- 2-bedroom: 1,200+ sq ft
- 3-bedroom: 1,600+ sq ft
- 4-bedroom: 2,000+ sq ft
- 5-bedroom: 2,500+ sq ft
Clay soil can double or triple required drainfield size. This is why soil type matters so much.
Reserve Areas: Your Expansion Insurance
When original system was installed, permits required designating “reserve area” - space for future drainfield expansion equal to original drainfield size.
Got 800 sq ft drainfield? Should have 800 sq ft reserve area marked on property.
Problem: Many older systems don’t have documented reserve. Or it exists on paper but you’ve built on it. Or landscaped over it. Or put in pool.
Solution when reserve is compromised:
- Use any remaining available space
- Install alternative system requiring less space (chambers, aerobic treatment)
- Connect to sewer if available
- Reduce proposed addition scope
Can’t create reserve area where it doesn’t exist. If your property is maxed out, you’re limited on expansion.
When You Don’t Need Accessory Use Permit
To be fair, some situations don’t require permits:
No Plumbing Added
Built shop with no bathroom? No plumbing = no accessory use permit needed for septic purposes. (Might need building permits though.)
Remodeling Without Adding Capacity
Renovating existing bathroom, replacing fixtures, updating kitchen - no new wastewater capacity = usually no accessory use permit. Though you should verify with health district.
Temporary Connections
Guest parking their RV for 2 weeks with temporary hookup? Generally doesn’t require accessory use permit. But check district policies - some define “temporary” differently.
Non-Septic Discharge
Installing composting toilet in shop? Greywater-only system for utility sink? These might not trigger accessory use requirements, but you need to verify specific regulations and probably still need some type of approval.
Costs to Expect
Let’s talk real numbers:
Application and Evaluation
Permit fee: $200-$400 Inspector time: Included Review process: 2-4 weeks typically
If Expansion Needed
Test holes and evaluation: $500-$1,000 Small drainfield expansion (200 sq ft): $3,000-$6,000 Moderate expansion (500 sq ft): $6,000-$12,000 Complete replacement system: $12,000-$25,000 Aerobic treatment system: $18,000-$30,000
Alternative Solutions
Separate small system for shop: $8,000-$15,000 Sewer line extension: $10,000-$30,000+ depending on distance Engineered solutions: $20,000-$50,000
Factor these costs into your ADU/addition budget BEFORE starting construction.
Common Mistakes That Cost Money
After watching many people do this wrong:
Building First, Asking Later
“It’s just a bathroom, I’ll get the permit after.” Then inspection shows system can’t handle it. Now you’ve built something you can’t legally use.
Cost: Removing unpermitted plumbing, upgrading system, retrofitting work. Often 2-3x what doing it right would’ve cost.
Assuming Capacity Exists
“My neighbor added an ADU so I can too.” But your neighbor might have better soil, larger reserve, or upgraded their system. Every property is different.
Cost: Wasted planning and expectations, plus full septic upgrade if you proceed.
Not Documenting Original System
“I think it’s a 1,000-gallon tank, installed maybe 1985?” Without documentation, health district might require you to prove capacity through pumping or even exploratory excavation.
Cost: $500-$2,000 to locate and document existing system.
Ignoring Reserve Area Setbacks
Built deck over your reserve area. Put in swimming pool. Paved it. Now can’t use it for expansion.
Cost: Either remove obstacle ($5,000-$20,000) or install new system elsewhere ($15,000-$30,000).
Skipping Plan Review
Start building before permit approval. Plan gets denied or requires modifications. Now you’re tearing out work.
Cost: Demolition and rework easily runs $5,000-$15,000.
Working With Health Districts
Make the process smoother:
Pre-Application Consultation
Call health district before submitting. Explain what you want to do, ask if it seems feasible. They’ll give preliminary guidance (not approval, but direction).
Many districts encourage this. It saves everyone time.
Complete Application First Time
Incomplete applications get sent back. That delays everything and frustrates everyone. Use their checklist, provide everything requested.
Be Honest About Plans
Don’t call your 2-bedroom ADU a “storage building with bathroom.” Inspectors aren’t stupid. Dishonesty gets you denied and possibly reported for code violations.
Have Documentation Ready
Original septic permit, as-built, property survey, building plans - gather everything before starting. Searching for documents mid-process delays approval.
Respect the Process
Yeah, it seems bureaucratic. But health districts are protecting groundwater and public health. Work with them, not against them.
ADU-Specific Considerations
ADUs are hot topic in Idaho as housing costs rise. Special considerations:
City Zoning vs Septic Capacity
City might allow ADUs by zoning. But if your septic can’t handle it, you’re stuck. Check both zoning AND septic capacity early.
Deed Restrictions
Some subdivisions prohibit ADUs regardless of septic capacity. Check CCRs before planning.
Rental Regulations
Some cities require owner occupancy of either main house or ADU if renting. Research local rules.
Future Sale Impact
ADUs can increase property value if done right. But unpermitted or septic-constrained ADUs can kill sales or reduce value.
The Bigger Picture
Accessory use permits exist because:
- System Protection: Overloading septic kills it prematurely
- Groundwater Protection: Failed systems contaminate aquifers
- Property Value: Properly permitted work holds value
- Liability Protection: Unpermitted work is your liability risk
That $300 permit feels annoying until your overloaded system fails and you’re looking at $25,000 replacement plus contamination cleanup.
Timeline Expectations
Simple approval (adequate capacity): 2-4 weeks Expansion required: 4-8 weeks New system needed: 8-16 weeks Appeals or variances: Add 2-6 months
Plan accordingly. Don’t schedule construction start dates until you have approval in hand.
What Approval Actually Means
Getting accessory use permit approved means:
- Your proposed addition fits within system capacity
- Any required upgrades are identified
- You can proceed with construction
- Future inspections will verify compliance
Doesn’t mean you can skip other required permits (building, electrical, plumbing). Septic approval is just one piece.
Long-Term Maintenance Considerations
Adding accessory use increases system loading. This means:
More frequent pumping: Instead of every 3-5 years, maybe every 2-3 years More vigilant maintenance: Watch for signs of problems Higher failure risk: More use = more stress on system Shorter lifespan: System may need replacement sooner
Budget for increased maintenance costs when planning accessory use projects.
Bottom Line on Accessory Use and Septic
That ADU, shop bathroom, or guest house is doable IF your septic can handle it. The accessory use permit process exists to make sure it can.
Do it right:
- Research requirements early
- Document existing system capacity
- Apply for accessory use permit before building
- Upgrade system if needed
- Build knowing you’re legal and protected
Do it wrong and you’re looking at expensive fixes, potential fines, and possibly removal of unpermitted work.
The permit costs $300. Fixing it later costs $30,000. Easy math.
And that ADU rental income or shop convenience? It’s only worth it if your septic system doesn’t fail underneath it all. Plan capacity before planning construction.
Your septic system is the foundation of your property’s viability. Treat it with respect, maintain capacity margins, and that accessory use will serve you well for decades. Overload it and… well, you really don’t want to find out what overloaded septic systems do to properties.
Now go forth and add that ADU. Just do it with proper permits and adequate capacity. Future you will thank present you for doing it right.
Need Professional Help?
Our expert team is ready to help with all your septic needs in the Treasure Valley.