How to Build an ADU That Passes Inspection the First Time

Why First-Time ADU Inspection Passes Matter

Every ADU construction project in California requires multiple inspections throughout the building process. Each inspection verifies that the work completed meets California building code requirements. When an inspection fails, it means stopping work, making corrections, scheduling a re-inspection, and waiting for the inspector to return. This cycle of failed inspections and re-inspections is one of the most common causes of construction delays and budget overruns in ADU projects.

A single failed inspection typically delays your project by 1 to 3 weeks and costs $500 to $2,000 when you factor in the re-inspection fee, contractor downtime, and any correction work required. Multiple failed inspections can add months to your timeline and thousands of dollars to your budget.

The good news is that most inspection failures are entirely preventable. They result from common, well-known issues that experienced contractors know how to avoid. This guide walks you through every major inspection point in a California ADU project, identifies the most common failure reasons, and provides specific strategies for passing each inspection on the first attempt.

The ADU Inspection Sequence

A typical California ADU project requires 6 to 10 inspections, depending on the scope of work and local requirements. Understanding the sequence helps you plan and prepare for each inspection.

Inspection	When It Happens	What Is Checked
1. Foundation/footing	Before concrete is poured	Excavation depth, rebar placement, soil conditions
2. Slab/underfloor	Before slab pour or subfloor	Plumbing under slab, vapor barrier, rebar
3. Framing	After framing, before insulation	Structural framing, shear walls, hardware
4. Electrical rough-in	After wiring, before drywall	Wire routing, box placement, grounding
5. Plumbing rough-in	After piping, before drywall	Pipe sizing, venting, pressure test
6. Mechanical rough-in	After HVAC install, before drywall	Ductwork, refrigerant lines, venting
7. Insulation	After insulation, before drywall	R-values, installation quality, air sealing
8. Drywall nailing	After drywall hung, before taping	Fastener spacing, fire-rated assemblies
9. Final inspection	After all work is complete	Everything: fixtures, finishes, safety, code compliance

Top 15 Reasons ADU Inspections Fail

Based on data from California building departments and experienced ADU contractors, these are the most common reasons ADU inspections fail, along with how to prevent each one.

1. Missing or Incorrect Hardware Connections

What fails: Simpson Strong-Tie connectors, hold-downs, hurricane ties, or other structural hardware are missing, incorrectly installed, or the wrong size for the connection.

How to prevent it: Create a hardware schedule from the structural plans and check off each connection as it is installed. Have the framing crew do a pre-inspection walkthrough specifically checking hardware before calling for the framing inspection.

2. Inadequate or Missing Fire Blocking

What fails: Fire blocking is required at specific locations to slow the spread of fire within wall cavities and between floors. Common missed locations include the top of walls at ceiling level, around pipes and ducts that penetrate fire-rated assemblies, and at stairway openings.

How to prevent it: Reference the approved plans for fire blocking locations and install blocking before calling for framing inspection. Use appropriate materials (solid wood, plywood, or fire-rated caulk for small penetrations).

3. GFCI Protection Missing or Incomplete

What fails: Ground Fault Circuit Interrupter (GFCI) protection is required in kitchens, bathrooms, laundry areas, outdoor outlets, and any receptacle within 6 feet of a water source. Inspectors frequently find missing GFCI protection, especially in garage conversions where existing outlets were not upgraded.

How to prevent it: Install GFCI breakers or GFCI receptacles at every required location per the current NEC and California Electrical Code. Test each GFCI device before the inspection.

4. Incorrect Plumbing Venting

What fails: Every plumbing fixture requires proper venting to maintain trap seals and prevent sewer gas from entering the living space. Common failures include undersized vent pipes, vents that do not extend through the roof (or that terminate in an attic), and missing vents on fixtures.

How to prevent it: Follow the approved plumbing plans exactly. Verify that each fixture has proper venting per the California Plumbing Code. Use air admittance valves (AAVs) only where specifically allowed by your jurisdiction.

5. Insulation Not Meeting R-Value Requirements

What fails: California's Title 24 energy code specifies minimum insulation R-values for walls, ceilings, and floors based on climate zone. Inspectors verify that the installed insulation meets or exceeds these requirements. Common failures include wrong R-value, compressed insulation (which reduces effective R-value), and gaps or voids in insulation coverage.

How to prevent it: Reference the Title 24 compliance documents (CF-1R form) for exact R-value requirements. Ensure insulation fills cavities completely without compression or gaps. Use faced insulation with the vapor barrier oriented correctly.

6. Smoke and CO Detector Issues

What fails: California requires hardwired, interconnected smoke detectors in every bedroom, outside each sleeping area, and on every level. Carbon monoxide detectors are required outside sleeping areas and near any fuel-burning appliance. Missing detectors, battery-only detectors (instead of hardwired), or detectors in wrong locations are common failures.

How to prevent it: Install hardwired, interconnected smoke/CO detectors at all required locations. Test all devices before the final inspection. Keep the approved plans on-site showing detector locations.

7. Egress Window Non-Compliance

What fails: Every bedroom requires at least one egress window (an operable window large enough for emergency escape). The window must have a minimum 5.7 square feet of clear opening, a minimum height of 24 inches, a minimum width of 20 inches, and a sill height no more than 44 inches above the floor.

How to prevent it: Verify egress window dimensions during framing (before windows are ordered) and again during the window installation. Measure the clear opening, not the rough opening or the overall window size.

8. Improper Grounding and Bonding

What fails: All electrical systems must be properly grounded and bonded per the NEC. Common failures include missing equipment grounding conductors, missing grounding electrode conductor, and improper bonding of water pipes and gas lines.

9. Duct Leakage Testing Failure

What fails: If your ADU uses a ducted HVAC system, California requires duct leakage testing. The total duct leakage must be below a specific threshold (typically 6% of nominal airflow for new construction). Poorly sealed duct connections and register boots are common failure points.

10. Waterproofing in Wet Areas

What fails: Shower pans, tub surrounds, and bathroom floors must be properly waterproofed before tile or other finish materials are installed. Inspectors test shower pans by filling them with water and checking for leaks. Failed waterproofing means tearing out tile work and starting over.

How to prevent it: Use proven waterproofing systems (such as Kerdi membrane or liquid-applied membrane) installed per manufacturer instructions. Test the shower pan before tiling by filling with water for 24 hours and checking for leaks.

11-15. Additional Common Failures

• 11. Missing address numbers: The ADU must have visible address numbers per local fire code. Install them before final inspection.
• 12. Improper stair construction: If your ADU has stairs, riser height, tread depth, handrail height, and guardrail spacing must meet code. Measure carefully during framing.
• 13. Incomplete exterior finish: Exterior siding, stucco, or other cladding must be complete with proper flashing at all penetrations and transitions.
• 14. Missing or inadequate ventilation: Bathrooms need exhaust fans (50 CFM minimum), kitchens need range hoods, and the overall unit needs adequate natural ventilation (operable windows equal to 4% of floor area).
• 15. Non-compliant handrails and guardrails: Handrails must be graspable (1.25 to 2 inch diameter), and guardrails must be at least 36 inches high with balusters spaced no more than 4 inches apart.

Strategies for First-Time Inspection Success

1. Hire Experienced Subcontractors

The quality of your subcontractors directly determines your inspection success rate. Experienced plumbers, electricians, and framers who regularly work on ADU projects in your jurisdiction know what local inspectors look for and build to pass. When evaluating contractors, ask about their inspection pass rate and request references from recent ADU projects.

2. Keep Approved Plans On-Site

The inspector compares the actual construction to the approved plans. Make sure a complete, stamped set of approved plans is always on-site and that the work matches those plans exactly. If any changes are needed during construction, get them approved through a plan revision before the next inspection.

3. Do Your Own Pre-Inspection

Before calling for any inspection, walk through the work yourself (or have your general contractor do it) with a checklist of what the inspector will be looking at. This "pre-inspection" catches obvious issues that would otherwise result in a failed inspection and wasted time.

4. Communicate with the Inspector

Building inspectors are generally willing to help you succeed. If you are unsure about a specific code requirement, call the building department before the inspection and ask for clarification. Many inspectors appreciate proactive communication and will provide guidance that helps you avoid common mistakes.

5. Document Everything

Take photos of all concealed work before it is covered up. Document rebar placement, hardware connections, insulation installation, and plumbing and electrical rough-in. These photos can resolve disputes and serve as a reference if questions arise during later inspections.

Frequently Asked Questions About ADU Inspections

How much does it cost when an ADU inspection fails?

A failed inspection typically costs $500 to $2,000 when you factor in the re-inspection fee ($100 to $300), contractor labor to make corrections ($200 to $1,000+), and project delay costs. Multiple failed inspections compound these costs and can add significant time and expense to your project.

How long does it take to schedule a re-inspection?

In most California cities, you can schedule a re-inspection within 3 to 10 business days after making the required corrections. In busy jurisdictions like Los Angeles, wait times may be longer. Each re-inspection delay pushes back your entire project timeline.

Can I be present during my ADU inspection?

Yes. You or your contractor can (and should) be present during inspections. Being present allows you to ask questions, get immediate feedback on any issues, and understand what corrections are needed if the inspection does not pass. It also shows the inspector that you are engaged and committed to doing the work correctly.

What happens after the final ADU inspection passes?

After the final inspection passes, the building department issues a Certificate of Occupancy (or equivalent approval) for the ADU. This document confirms that the ADU was built according to the approved plans and meets all applicable building codes. You can then legally occupy the ADU or begin renting it to tenants.

Do ADU inspections differ from regular home inspections?

ADU inspections follow the same building code requirements as any other dwelling unit construction. However, inspectors may pay particular attention to fire separation (for attached ADUs and JADUs), utility connections (ensuring proper separation from the main house systems), and compliance with ADU-specific requirements like parking provisions and setbacks.

What if I disagree with the inspector's findings?

If you believe the inspector made an error or is applying the code incorrectly, you can request a review by a senior inspector or the building official. Approach this diplomatically and bring the specific code sections that support your position. In most cases, legitimate disagreements can be resolved through a collaborative discussion with the inspection department.

Pre-Inspection Checklist by Trade

Preparing for ADU inspections systematically by trade ensures nothing is overlooked. Use this checklist before calling for each inspection to increase your chances of passing on the first attempt.

Foundation and Grading Inspection Checklist

• Excavation depth matches engineered plans exactly
• Soil bearing capacity has been verified per the geotechnical report
• Formwork is level, square, and properly braced
• Rebar is the correct size, spacing, and placed at the specified depth with proper cover
• Anchor bolt locations match the approved framing plan
• Hold-down anchors are installed at all required locations
• Grading directs water away from the foundation on all sides
• Approved plans and soils report are available on-site for the inspector

Framing Inspection Checklist

• All lumber sizes match the approved structural plans
• Stud spacing matches the plan (typically 16 inches on center)
• Headers over windows and doors are the correct size and properly supported
• Shear wall nailing patterns match the structural engineering specifications
• Hold-down straps and connectors are installed at all required locations
• Roof framing matches the approved plans including rafter size and spacing
• Fireblocking is installed at all required locations (floor/ceiling junctions, stair enclosures)
• Window and door rough openings match the sizes specified on the plans
• Approved plans are posted and accessible on-site

Electrical Rough-In Inspection Checklist

• Panel is properly sized and located per the approved electrical plan
• All circuits are the correct wire gauge for their amperage rating
• GFCI protection is provided in bathrooms, kitchens, garages, and outdoor locations
• AFCI protection is provided in bedrooms and living areas (required in California)
• Smoke detectors are hardwired with battery backup in every bedroom, hallway, and living area
• Carbon monoxide detectors are installed per California requirements
• Outlet spacing meets code (no point along a wall more than 6 feet from an outlet)
• Dedicated circuits are provided for kitchen appliances, bathroom, and laundry
• All wire is properly secured and supported within required distances

Plumbing Rough-In Inspection Checklist

• All drain pipes are the correct diameter for their fixture type
• Drain slope is minimum 1/4 inch per foot throughout the entire run
• Every fixture has proper venting per code
• Water supply lines are the correct size and properly supported
• Shut-off valves are accessible at every fixture location
• Shower pan has been flood-tested and documented as leak-free
• Water heater location meets clearance and venting requirements
• Cleanouts are installed at required locations for drain maintenance access

Mechanical (HVAC) Inspection Checklist

• Equipment size matches the Manual J load calculation and Title 24 energy report
• Ductwork is properly sized, sealed, and insulated per Title 24 requirements
• Combustion air is provided for any gas appliances
• Exhaust fans in bathrooms and kitchens vent to the exterior (not into the attic)
• Return air pathways are adequate for proper air circulation
• Refrigerant lines are properly sized and insulated

Common ADU Inspection Failures and How to Avoid Them

Even experienced contractors sometimes fail inspections. Understanding the most common failure points helps you and your builder prepare more effectively. Here are the inspection failures California ADU inspectors see most frequently:

Missing or Incorrect Fire Separation

When an ADU is attached to a garage or within a certain distance of the main home or property line, fire-rated construction is required. Common failures include missing 5/8-inch Type X drywall on garage-adjacent walls, unsealed penetrations through fire-rated assemblies (every pipe, wire, and duct that passes through a fire-rated wall must be fire-caulked), and missing self-closing devices on doors between the ADU and a garage space.

Inadequate Insulation and Air Sealing

California's Title 24 energy code is strict, and inspectors verify insulation values and air sealing thoroughly. Common failures include insulation that does not fill the cavity completely (gaps and compression reduce R-value), missing insulation at rim joists, band joists, and behind plumbing or electrical in exterior walls, and inadequate air sealing around windows, doors, and penetrations through the building envelope.

Electrical Code Violations

Electrical inspection failures frequently involve missing GFCI or AFCI protection where required, incorrect wire gauge for the circuit breaker size, junction boxes that are not accessible (buried behind drywall or insulation), and insufficient number or incorrect placement of outlets, particularly in kitchens where specific counter-space outlet rules apply.

Plumbing Venting Issues

Improper plumbing venting is one of the most common causes of plumbing inspection failures. Issues include fixtures that are too far from the vent stack without proper re-venting, vent pipes that terminate too close to windows, doors, or air intake openings, and incorrect vent pipe sizing for the number and type of fixtures served.

Egress Window Non-Compliance

Every bedroom in a California ADU must have an emergency escape and rescue opening (egress window or door). The minimum opening must be at least 5.7 square feet (5.0 square feet on ground floor), with a minimum width of 20 inches and minimum height of 24 inches. The bottom of the opening must be no more than 44 inches above the floor. Inspectors frequently fail windows that are close to but do not quite meet these minimums, so verify measurements carefully before calling for inspection.

Frequently Asked Questions About ADU Inspections

How many inspections does an ADU require in California?

A typical ADU project in California requires 8 to 15 separate inspections depending on the scope of work and your local jurisdiction. Common inspections include foundation/footing, underground plumbing, slab pre-pour, framing, electrical rough-in, plumbing rough-in, mechanical rough-in, insulation/energy, drywall nailing, and final inspection. Some cities combine certain inspections while others add additional checkpoints. Your approved permit card will list every required inspection for your specific project.

What happens if my ADU fails an inspection?

If your ADU fails an inspection, the inspector will provide a correction notice listing the specific items that need to be fixed. You or your contractor must correct all listed items and then schedule a re-inspection. There is typically no additional fee for the first re-inspection, but subsequent re-inspections for the same items may incur fees ($75 to $200 per visit in most California cities). Failed inspections delay your project timeline, so addressing correction items promptly is important. In most cases, corrections can be completed within a few days.

Can I schedule multiple ADU inspections on the same day?

Yes, many California building departments allow you to schedule multiple inspections for the same day if the work for each inspection is complete and ready. For example, you might schedule electrical rough-in, plumbing rough-in, and mechanical rough-in inspections on the same day since these trades work concurrently. However, some inspections must occur sequentially. Foundation inspections must pass before concrete is poured, and rough-in inspections must pass before insulation and drywall can be installed. Contact your building department to coordinate efficient inspection scheduling.

Do I need to be present for ADU inspections?

You or your authorized representative (such as your general contractor) should be present for all inspections. While some jurisdictions allow inspections with no one on site as long as the inspector has access, having a knowledgeable person available to answer questions and provide documentation speeds the process. Your contractor should have the approved plans, engineering documents, and any required test results (such as plumbing pressure tests) available for the inspector to review on demand.

How do I prepare for the final ADU inspection?

The final inspection is the last step before your ADU receives a certificate of occupancy. To prepare, ensure all previous inspection corrections have been completed and signed off, all fixtures and appliances are installed and functional, smoke detectors and carbon monoxide detectors are installed and operational, address numbers are visible from the street, all required guardrails and handrails are in place, the site is graded properly with drainage away from the structure, and all required landscaping or hardscaping is complete per the approved plans. Having a clean, organized site with clear access to all areas of the ADU makes the final inspection go smoothly. For a complete overview of the ADU building process, see our guide on how long it takes to build an ADU in California.