If we did a fixed price construction contract before the plans were finalized, based on a drawing, but the plans were changed 100% by the homeowner and the price is astronomical because of the changes can we change our construction contract. We cannot build their plan for the price we agreed
As a practical matter, this is certainly an unfortunate, but not unforseeable, result of executing a fixed price based on incomplete drawings, considering the many additional specs that might be added before final plans. On the other hand, as a legal matter there are likely a number of theories and defenses that would allow you to negotiate for a change in price or rescision of the agreement, such as mutual mistake.
You would be best served by taking the contract to experienced construction counsel to review the agreement and specific facts, and get advise on best strategy to proceed. I wish you luck.