In many cases, you will need permission from the first builder to build a house based on the plan. This is because the builder is likely to own copyright in the plan, and you may not have an implied licence to build using another builder. You should seek legal advice if you are considering building from such a plan without the builder’s permission. If you decide not to build from that plan, it is advisable not to show that plan to anyone you subsequently ask to draw a new plan for you. If the second person does see the earlier plan there may be an inference that that person has consciously or subconsciously copied original elements of the earlier plan.