He would only be justified in doing so if she had committed a material breach in this case, if cutting down the trees and by building the house facing North constitute a material breach. Perhaps in view of the gap in the contract, the Contractor is entitled to use her best judgment. Also, perhaps the trees needed to be removed in order for excavation and other heavy equipment to prepare the foundation of the house. If the plans do call for solar panels or solar windows, Betty should realize that the house has to face South to utilize them. If she was in doubt, she should have called Oscar to clarify these points.
Facts are a bit sketchy to know for sure who breached.
It is a close issue. The goal based on expectation is for the owner to end up with a completed building for a cost of no more than the contract price. But where the cost of completion or of replacing defective performance is significantly out of proportion to the increase in value which results, courts sometimes refuse to award cost of completion. However, not all courts agree. But Betty may argue that there will be economic waste in order to change the direction of the house, so Oscar s argument is not certain to succeed.