Yes: even with no design duty, a contractor owes an implied duty to work with the skill and care of a reasonably competent builder, so it can be liable for building to a drawing whose error a competent builder would have spotted, subject to a reduction for the employer contributory negligence

Yes. A contract that appoints an architect and says nothing about contractor design does not mean the contractor cannot be liable for a design fault. Every contractor owes an implied duty to work in a good and workmanlike manner with the care of a reasonably competent builder, and where the drawings contain an error a competent builder would have spotted, following them blindly is no answer. A builder who demolished walls shown on the plans as non-load-bearing was held liable in breach of that duty because, as a competent builder, he should have known nine-inch walls were load-bearing and should have supported them. His liability was reduced heavily for the client's contributory negligence, since the client's agents had supplied the wrong plans. A variation can also impose a design responsibility where it requires the contractor to work out how the work is to be done.