You said yourself that the builder agreed to accept 10k for the work.  If the work costs the builder more than that, through no fault of the customer, then the builder has screwed up and gets to eat the loss.  The builder is lucky the customer paid some extra, because there was no legal reason he could be forced to do so.\n\nThe builder needs to be more careful about the deals he makes, make sure he estimates job costs better, and maybe include something in the agreement about handling cost overruns in the future.