Short Answer
Before hiring an ADU builder, review their completed ADU project portfolio, call at least two past clients from similar project types, verify current insurance certificates directly, compare bids line by line across identical scope, and read every clause in the contract before signing. This process takes a few hours and can save months of problems.
Review Their Completed ADU Portfolio
A portfolio tells you more than a license number. Ask each contractor for photos or a site visit to at least two completed ADU projects from the past two years.
What to look for:
Project type match: If you are building a detached ADU, look for completed detached ADU projects — not just garage conversions. The structural work, permitting path, and utility connections differ meaningfully between project types.
Quality of finishes: Tile work, trim, cabinetry, and fixtures reveal craftsmanship. Basic finishes done carefully signal attention to detail across the whole project.
Permit history: Ask whether each project was fully permitted and passed final inspection without major corrections. A contractor who gets clean plan check approvals saves you time and money.
Some contractors maintain a portfolio on their website. Others can provide photos or addresses of recent projects. If a contractor cannot show you any completed ADU work specifically, that is a data point worth weighing.
Call Their References — With a Script
Online reviews matter, but a direct reference call gives you information you cannot find anywhere else. Ask for two or three clients from completed ADU projects, ideally within the past 18 months.
When you call, ask these questions:
- 1
Did the project finish on time, and if not, what caused the delay?
- 2
Did the final cost match the original bid, or were there significant change orders?
- 3
Did the contractor pull permits without problems, and did the work pass inspections the first time?
- 4
Was communication consistent — or did you have to chase them for updates?
- 5
Would you hire them again?
The last question tends to produce the most honest answer. A satisfied client says yes without hesitation. A client who had serious problems will often hedge or give a qualified response — which is worth following up on.
Verify Insurance Directly
Do not accept a verbal confirmation that a contractor is insured. Ask for a current certificate of insurance that shows:
General liability insurance: Covers property damage or injury during construction. For ADU projects, a minimum of $1 million per occurrence is standard in California.
Workers' compensation insurance: Required in California for any contractor with employees. Without it, you may be liable if a worker is injured on your property during the project.
The certificate should list your property address and show a current expiration date. If the policy lapses mid-project, your coverage gap becomes your problem.
You can also verify on cslb.ca.gov whether a contractor has a workers' compensation exemption on file (applicable to sole proprietors with no employees). This is separate from the license check and takes only a moment.
Note: Insurance requirements can vary by project scope and contract structure. Consult a qualified insurance professional to understand your specific liability exposure before construction begins.
Compare Bids Line by Line
Getting multiple bids is standard advice. Comparing them correctly is less common. Before looking at totals, confirm that each bid covers the same scope of work.
Ask each contractor to break their bid into these categories:
Design and plan preparation (if included)
Permit fees and city processing fees
Site preparation and demolition
Foundation or slab work
Framing, roofing, and exterior
Electrical, plumbing, and HVAC
Insulation and drywall
Interior finishes (kitchen, bath, flooring)
Contingency allowance
A bid that is $30,000 lower may simply be missing line items the other contractors included. Once you align scope, the comparison becomes genuinely useful. The San Diego ADU cost guide provides reference ranges for each cost category if you want a benchmark.
Watch for the contingency line. An honest contractor includes a contingency — typically 10–15% of hard construction costs — to account for conditions found once walls open or footings are dug. A bid with no contingency is either incomplete or unrealistically optimistic.
Read the Contract Before Signing
The contract is your protection if anything goes wrong. A one-page agreement for a six-figure ADU project is not adequate. A proper ADU contract should include:
Scope of work: Specific materials called out by type, brand, or grade — not vague language like 'standard finishes.'
Payment schedule tied to milestones: Payments should be triggered by completed construction stages, not calendar dates. This protects you if the project stalls.
Change order process: All scope or cost changes must be in writing before work proceeds, with cost and timeline impact documented. Verbal change orders are not binding.
Permit responsibility: Specifies who pulls each permit and who pays for plan check resubmittals if corrections are required.
Lien waiver provisions: Requires the general contractor to provide documented proof that subcontractors and suppliers have been paid before each payment milestone. This protects you from mechanics liens filed by subs the GC fails to pay.
Warranty terms: Specifies the duration and coverage of the contractor's warranty on labor and materials.
Termination clause: Defines what happens if the project stalls significantly or either party needs to exit the agreement.
California law requirement: California Business and Professions Code Section 7159 sets mandatory elements for home improvement contracts, including a cooling-off period for contracts signed away from the contractor's place of business. Any contract worth signing will already meet these requirements. One that does not is a red flag.
Homeowner Vetting Checklist
Before signing a contract with any ADU builder:
Reviewed portfolio of completed ADU projects matching your project type
Called at least two past ADU clients directly using the reference script
Received and verified a current certificate of insurance (general liability + workers' comp)
Confirmed active license on cslb.ca.gov with correct class and no open complaints
Received at least three itemized bids covering identical scope
Compared bids line by line — not just the total
Confirmed each bid includes a contingency line item (10–15% of hard costs)
Read the full contract: scope, payment milestones, change order process, lien waivers
Confirmed the contractor pulls all required permits in their own name
Verified the contractor is current on California ADU laws since 2020
Final Thoughts
Vetting a contractor properly takes a few hours spread across a few days. Most homeowners who skip this process do so because they feel pressure to decide quickly. That pressure often comes from the contractor — which is itself worth noticing.
A contractor who clears a thorough vetting process is far more likely to deliver a project that finishes close to the original budget, passes inspections cleanly, and holds up over time.
To compare ADU builders in San Diego by project type and service area, browse the San Diego ADU builder directory, or get matched with qualified ADU professionals based on your specific project and location. For guidance on the permit side of the project, the San Diego ADU permit guide explains what to expect from plan check to final inspection.
Frequently Asked Questions
How is vetting different from finding a contractor?+
How many references should I call before hiring an ADU builder?+
What should I do if a contractor refuses to provide references?+
Is it normal for ADU contractor bids to vary widely?+
What is a mechanics lien and why does it matter for ADU projects?+
Can I negotiate the contract terms with an ADU builder?+
Disclaimer: This article is for informational purposes only and does not constitute legal, zoning, permitting, contracting, insurance, financial, engineering, or construction advice. Requirements vary by property, city, and project type. Consult a licensed professional before making decisions about your ADU project.
Sources and Official Resources
- •California Contractors State License Board — Verify a License — cslb.ca.gov. Accessed June 2026.
- •California Contractors State License Board — Workers' Compensation Insurance Requirements — cslb.ca.gov. Accessed June 2026.
- •California Business and Professions Code Section 7159 — Home Improvement Contracts — leginfo.legislature.ca.gov. Accessed June 2026.
- •California Department of Housing and Community Development — Accessory Dwelling Units — hcd.ca.gov/policy-research/accessory-dwelling-units. Accessed June 2026.
California contractor licensing and insurance requirements can change. Verify current requirements with the Contractors State License Board and consult qualified professionals before hiring.