I am in Ohio and I don't know anything about Utah, but here, almost anyone in construction would know that a floor plan and plot plan are filed to get a building permit. These blueprints can be viewed and copied by the public and are open to the public.
I think the police in the Smart case are clueless, and they floated the story that only someone that knew the family or a family member might be involved because the abductor knew where the child slept. I have also noticed they are not stressing that point any longer.
I would construct a list of everyone that has ever been in that house. Since the father built the house and contracted the work out he could put a list together easily enough. The idea that this was a staged elopement can't be overlooked as well.