ADU problems
ADU cost overrun: why it happens and what to do
By the HyreADU research desk. Last checked .
The short answer
An ADU cost overrun usually starts in a few places: site work and utility connections that were left out or underestimated, low allowances for finishes, unknown conditions in a conversion, and change orders.
In California, change orders must be written and signed before the work, so ask for every change in writing, compare it with the contract, and pay only for completed work.
Why it happens
- Site work, trenching and utility connections missing from the bid or carried as small allowances.
- Allowances for cabinets, tile or fixtures set too low.
- Hidden conditions in a conversion: a slab, framing or wiring that must be replaced.
- Plan check corrections that change the design.
- Owner-requested changes during the build.
- Utility connection or capacity charges, which are not impact fees and are not waived by California’s 750 sq ft rule.
What to do this week
- List every cost increase so far, with the reason given.
- Match each one to a signed change order. California says change orders become part of the contract once written and signed before the work (Bus. & Prof. Code § 7159).
- Ask the builder for an updated cost-to-complete in writing.
- Check that payments made so far do not exceed the work done (§ 7159.5).
- Decide what can be cut or deferred: finishes, landscaping, extras.
Who fixes it
The builder prices changes; you approve or refuse them. A designer can suggest cheaper equivalents. If a dispute grows, the CSLB handles complaints about licensed contractors; we give no legal advice. Read ADU change orders.
What it costs and how to prevent it next time
The cost is the increase itself plus delay. We publish no amounts. Prevention is a complete, itemized bid with site work and utilities as lines, realistic allowances, and a contingency you set aside yourself. The ADU bid comparison checks bids for those lines, and the bid red flags page lists what to watch for.
A mistake to avoid
The mistake to avoid is approving extra work on site by word of mouth. Once the work is done, the price is hard to argue. Ask for the written change order first, even for small items, and keep a running total.
What to do next: check your lot with the ADU feasibility checker, read the questions to ask an ADU builder, then tell us about your project. Matching is not live yet, so we reply with information, not a quote.
Get ADU optionsQuestions
Can a contractor charge more than the contract price?
Only through written, signed change orders in California, or under the contract’s own terms. We give no legal advice on disputes.
What is a contingency?
Money you hold back for surprises. It is your budget line, not the builder’s.
Are utility charges part of impact fees?
No. Connection and capacity charges are separate and can still be billed.
Should I stop paying?
Pay for completed work under the contract. For disputes, get legal advice.
Sources
- Bus. & Prof. Code § 7159 (home improvement contract contents, change orders). Checked September 5, 2026.
- Bus. & Prof. Code § 7159.5 (down payment cap, no payment ahead of work). Checked September 5, 2026.
- Gov. Code § 66311.5 (ADU impact fees, connection fees and capacity charges). Checked September 5, 2026.
- CSLB Check a License (search by license number, business name, personnel name or HIS registration). Checked October 27, 2026.
HyreADU is a publisher. It does not design, permit or build ADUs, has no partner builders, and gives no legal, tax, lending or engineering advice.
Rules vary by state and city; California rules are labeled as such. Spotted an error? Tell us through our corrections page. Licensed professionals can also contribute as named sources.