Key legal question
Whether the additional work orders created binding supplementary contracts with the owner, not with the house owners.
Extracted holding
The additional orders were validly placed through the construction supervision acting for the owner, so the owner is bound.
Extracted reasoning
The owner had confirmed that C.________ was also responsible for construction supervision. The contractor had announced the extra work in advance and received express approval from C.________ or, in one case, D.________. The later signatures by A. Z.________ were only for evidentiary purposes and did not shift the contractual counterparty.