Change orders: written, priced and signed before the extra work, or it is a gift
In one paragraph
Behind the drywall is where the contract price changes. California's answer is one sentence with three parts: a change order becomes part of a home improvement contract only if it is in writing and signed by the parties before the changed work begins, and it is unenforceable against the customer unless it states the scope, the dollar amount added or subtracted, and the effect on the payment schedule and completion date. Say it on the phone and do the work, and you are relying on the customer's goodwill for the money. Here is the two-minute habit that makes extras collectible.
The rule
A change-order form must be incorporated into the home improvement contract, and a change becomes part of the contract only if it is in writing and signed by the parties before the commencement of the work it covers (B&P 7159(c)(5)). The contract must tell the buyer that a change order is not enforceable unless, in writing and before the work, it identifies the scope, the amount to be added or subtracted, and the effect on progress payments or the completion date (7159(e)(3)); the contractor's failure to comply does not preclude recovery on legal or equitable theories designed to prevent unjust enrichment, but that is a lawsuit, not an invoice.
Why it matters
Extras are where the margin lives on a remodel and where the disputes live afterward. A written change order signed before the work is the difference between a line item the customer already agreed to and an argument in the driveway about what 'you said around four hundred' meant. It also protects the customer, which is why the statute puts the notice in their contract and why CSLB reads it that way.