A common misconception within the building industry is that you can change 10% of a plan and copyright no longer applies. This is incorrect and many people have fallen victim to copyright infringement lawsuits.
For copyright infringement, the part reproduced need only be a "substantial part" of the original work, not the newly created work. The questions arises as to what constitutes a "substantial part". There are cases outside of the building industry that show that even taking a small part of someone's work and re-using it constitutes an infringement. There have also been cases where infringements occurred from small items, such as copying a unique Entryway, or facade.
The plans need not be formal plans either. Even an idea taken from a concept sketch, CGI model or even an email discussing ideas can constitute a copyright infringement.
Typically, builder's are very protective of their plans, as it reflects their work, ideas and image. Some companies will undertake background checks on people who made enquiries to ensure their plans have not been used or manipulated by a third party.