Legal Question in Construction Law in California

It's a little difficult to answer this question with any certainty without comparing the contract to the invoice for the additional charges and the description for those charges and knowing whether this was residential or commercial work. The law is a little more lenient on contractors for commercial work as compared to residential work.

The question I would need to answer, by comparing the contract to the description for the extra charges, is "is this truly the same exact work contemplated by the parties in the original contract, or is it "extra work" (i.e. 'change order(s) work') that modified the original contract price because of some difference (scope of work, materials, labor, etc.)?"

Generally, a contract should describe in detail the work to be performed and how any "change order" work will be handled. Most construction contracts (and the law) require change orders to be in writing and signed by the parties, describing the additional work to be performed, the change in cost (if any) to the base contract) and the effect, if any, on the deadline to complete the contract (i.e. any time extensions). If there were no change orders you signed approving those extra charges (or if you did not give any verbal approval such that the contractor detrimentally relied upon to its disadvantage) then they might not be valid, regardless of whether or not this truly was extra work performed by the contractor.

On the other hand, if it truly was "extra work" different from the original contract, and you approved such work, possibly even with just a verbal approval, then you very well could be responsible for such charges.

I would suggest you talk this over with the contractor to get his position on what the extra charges are for and, if you are not satisfied with the response, consult an attorney.