Utility Trenching
Your ADU needs connections to water, sewer, electricity, and possibly gas. Utility trenching involves excavating paths for these lines from the main house or street connections to your ADU location. Getting this right prevents future problems and ensures code compliance.
Quick Summary
Time needed
2-5 days
Cost
$3,000-10,000
Key contractors
Plumber & electrician
Why Utility Trenching Matters
Underground utilities must be installed to specific depths and standards. Improperly installed lines can lead to:
- Frozen water pipes in winter (too shallow)
- Sewer backups from improper slope
- Electrical hazards from inadequate burial depth
- Failed inspections requiring expensive re-work
Utility Line Requirements
Water Supply Line
Connects your ADU to the main house water supply or a separate meter.
Tip: Run a separate shutoff valve at the connection point for easy maintenance. Consider adding a pressure regulator if main pressure exceeds 80 PSI.
Sewer/Drain Line
Carries wastewater from ADU to main sewer connection or septic system.
Critical: Sewer slope must be precise. Too flat causes clogs; too steep causes solids to separate from liquids. Use a laser level.
Electrical Line
Provides power from main panel or new meter to ADU subpanel.
Code requirement: Direct burial cable (UF-B) must be 24" deep. Running in conduit allows shallower burial (18") and easier future upgrades.
Gas Line (if applicable)
Supplies natural gas for heating, water heater, or cooking. Optional for all-electric ADUs.
Consider: Many jurisdictions now require or incentivize all-electric ADUs. Heat pumps for HVAC and water heating eliminate gas costs entirely.
Trenching Process Step-by-Step
Plan Utility Routes
Work with your contractor to determine the best path for each utility:
- • Shortest route that avoids obstacles (trees, existing utilities)
- • Entry points through foundation
- • Maintain required separation between different utilities
- • Consider running multiple utilities in same trench where allowed
Excavate Trenches
Dig trenches to proper depths with appropriate equipment:
- • Mini excavator for longer runs or rocky soil
- • Trencher machine for straight, narrow trenches
- • Hand digging near existing utilities (within 24")
- • Stockpile excavated soil for backfill
Safety: Trenches over 4 feet deep require shoring or sloping to prevent cave-ins. OSHA regulations apply to all excavations.
Prepare Trench Bed
Create a stable, even surface for pipes and conduit:
- • Remove rocks and debris from trench bottom
- • Add 2-4" sand or pea gravel bedding for water/sewer lines
- • Ensure proper slope for sewer (1/4" per foot minimum)
- • Compact bedding material
Install Utilities
Lay pipes and conduit according to code requirements:
- • Water line first (often deepest)
- • Sewer line with proper slope and cleanouts
- • Electrical conduit with pull string inside
- • Gas line (if applicable) at required separation
- • Stub up through foundation forms
Schedule Inspection
Underground utilities must be inspected before backfilling:
- • Call for inspection when trenches are open
- • Inspector verifies depths, materials, slope
- • Pressure test water lines (typically 150 PSI for 15 min)
- • Do NOT backfill until inspector approves
Backfill and Compact
Properly backfilling prevents settling and protects utilities:
- • Cover pipes with 6" sand or fine soil first
- • Backfill in 6-8" lifts, compacting each layer
- • Install warning tape 12" above utilities
- • Finish grade to match surrounding area
Utility Trenching Cost Breakdown
Typical costs for 50-100 foot utility runs:
Common Trenching Problems
Rocky Soil
Rock excavation can triple trenching costs. Consider shallow burial with insulation or above-ground runs where permitted. Budget extra for rocky areas.
High Water Table
Groundwater filling trenches slows work and requires dewatering pumps. May need special pipe bedding or raised utility routing.
Long Distances
Runs over 100 feet significantly increase costs. Electrical may need larger wire to prevent voltage drop. Sewer may need cleanouts every 50 feet.
Separate Meter Requirements
Some jurisdictions require separate utility meters for ADUs. This adds $2,000-5,000+ for meter installation and connection fees.
Frequently Asked Questions
Can utilities share the same trench?
Yes, with proper separation. Water and electrical are often run in the same trench 12" apart horizontally. Sewer is usually separate due to depth and slope requirements. Gas must be 12-18" from other utilities. Check local codes.
Do I need a separate sewer connection?
Usually no. Most ADUs connect to the existing house's sewer line before it reaches the city main or septic. A new "wye" fitting is installed to add the ADU connection. Some cities require a separate tap for larger ADUs.
What about internet/cable?
Run empty conduit for future low-voltage wiring even if you're not installing now. A 1" conduit from house to ADU costs $100-200 during trenching but saves thousands if needed later. WiFi from main house works for many ADUs.
Should I go all-electric and skip gas?
Yes, in most cases. Modern heat pump HVAC and water heaters are efficient and eliminate gas trenching costs ($500-2,000). Many cities now require or incentivize all-electric new construction. Saves ongoing gas connection fees too.
Ready for the Next Step?
With utilities in place and trenches still open, the next step is scheduling your foundation inspection before pouring concrete.