Assess Water & Sewer Capacity
Your ADU needs reliable water supply and proper sewer drainage. Aging pipes, insufficient pressure, or overloaded sewer lines can create expensive problems. Getting this assessment early helps you budget accurately and avoid mid-project surprises.
Quick Summary
Water line cost
$1,500-4,000
Sewer line cost
$3,000-15,000
Sewer scope cost
$200-500
Why Water and Sewer Assessment Matters
Plumbing infrastructure is hidden underground and often forgotten until problems arise. Many ADU projects discover water or sewer issues after construction begins, leading to expensive emergency repairs. Common surprises include:
- Tree roots infiltrating old clay sewer pipes ($5,000-15,000 to replace)
- Low water pressure making showers and appliances unusable
- Sewer capacity issues causing backups in both units
- Permit rejections due to inadequate sewer lateral sizing
Understanding Water Requirements
Water Pressure Standards
How to Assess Your Water and Sewer Systems
Test Your Water Pressure
Buy a water pressure gauge ($10-15 at hardware stores) and attach it to an outdoor hose bib. Test at different times of day:
- • Morning (6-8 AM) - often highest demand in neighborhood
- • Afternoon (2-4 PM) - typically lower demand
- • Evening (6-8 PM) - high demand period
Pro tip: If pressure varies by more than 10 PSI throughout the day, or drops below 40 PSI at peak times, you may need a pressure booster for your ADU.
Get a Sewer Scope Inspection
A plumber runs a camera through your sewer lateral (the pipe from your house to the city main). This $200-500 inspection reveals:
- • Pipe material: Cast iron, clay, orangeburg (all problematic), or PVC (good)
- • Root intrusion: Tree roots growing into pipe joints
- • Bellies and sags: Low spots where waste accumulates
- • Cracks and breaks: Structural damage requiring repair
- • Pipe diameter: 3" minimum, 4" preferred for ADU
Important: Many jurisdictions require a sewer scope as part of ADU permitting. Even if not required, it's essential for budgeting accurately.
Check for Sewer Capacity Requirements
Some jurisdictions require proof that your sewer lateral can handle additional fixtures. You may need:
- • Fixture unit calculation: Each fixture has a "drainage fixture unit" value
- • Pipe sizing verification: 4" lateral typically handles 2 residential units
- • Sewer authority approval: Some cities require formal capacity verification
Your plumber or engineer can perform these calculations as part of permit drawings.
Evaluate Water Meter and Service Size
Your water meter and service line size affect flow rate to your ADU:
- • 5/8" or 3/4" meter: Standard for single-family, usually adequate for small ADU
- • 1" meter: Better for larger ADUs or high-flow fixtures
- • Service line: Should match or exceed meter size
Contact your water utility to verify your current service size and ask about upgrade costs if needed.
Plan the Water Line Route
New water supply to your ADU typically branches from your existing supply near the main house:
- • Trenching depth: Below frost line (varies by region, 18" to 48")
- • Pipe material: PEX or copper (PEX preferred for underground)
- • Shut-off valves: At main connection and ADU entry point
- • Insulation: Required in cold climates to prevent freezing
Design the Sewer Connection
Your ADU sewer line must connect to the existing lateral with proper slope:
- • Minimum slope: 1/4" per foot (2% grade)
- • Preferred slope: 1/8" to 1/4" per foot
- • Connection point: Wye fitting into existing lateral
- • Cleanout access: Required for maintenance
Note: If your ADU is downhill from the sewer connection, you may need a sewage ejector pump ($2,000-4,000 installed).
Water and Sewer Cost Breakdown
Common Water and Sewer Pitfalls
Ignoring Old Clay or Orangeburg Pipes
Homes built before 1970 often have clay, cast iron, or orangeburg (tar paper) sewer pipes. These are near end-of-life and should be replaced during ADU construction. Doing it later costs 2-3x more.
Underestimating Trenching Costs
Underground utility runs are expensive: $30-75 per linear foot including trenching, pipe, backfill, and restoration. A 50-foot run can cost $2,000-4,000. Plan routes carefully to minimize distance.
Forgetting About Hot Water
Long pipe runs between the water heater and ADU mean waiting for hot water and wasted water/energy. Consider a point-of-use water heater or recirculation system for runs over 30 feet.
Frequently Asked Questions
Can I connect my ADU to a septic system?
Yes, but it requires septic system evaluation and possibly expansion. A septic engineer must verify your system can handle additional flow (typically 100-150 gallons per bedroom per day). Septic expansion costs $5,000-20,000. Some jurisdictions require connecting to municipal sewer if available within a certain distance.
Do I need a separate water meter for my ADU?
Not usually required, but recommended for rentals. Separate water meters cost $1,000-3,000 and let tenants pay their own water bills. Some water utilities offer discounted ADU meter installation. Check with your local utility for requirements and costs.
What if my sewer scope shows problems?
Common issues and solutions: Minor root intrusion ($500-1,000 to clear), pipe lining for cracks ($3,000-6,000), full replacement for collapsed or severely damaged pipe ($8,000-15,000). Get quotes from 2-3 plumbers. Consider replacing now rather than risking problems after ADU is complete.
How do I know if my water pressure is sufficient?
Test with a gauge at an outdoor faucet during peak usage times. You need at least 40 PSI for code compliance and 50+ PSI for comfortable operation. If pressure drops significantly when running multiple fixtures, your supply line may be undersized. A plumber can evaluate and recommend solutions.
Ready for the Next Step?
Once you understand your water and sewer capacity, the next step is planning your HVAC approach to ensure comfortable year-round temperatures in your ADU.