Is it ethical to copy a house plan without purchasing the plan?

It is not only not ethical but illegal, and no architect will duplicate the house plan design if they know it is available for sale. You can use the design as inspiration, but an exact duplicate is not allowed. It is the same as pirating movies or music off the internet. You might want to check other house design websites or search online for that particular architect house plans, as other sites often carry the same thing but charge different amounts. If you just get the blueprint set it is usually less expensive than including the CAD files, which are generally used if you want to make minor changes to the plan. And even if you purchase this set of blueprints, you have to have a local architect review and approve, generally, as part of the building permit process.

your local building department will not approve the plans without an architect's signature that he personally did the required safety computations. that means that he's going to get paid.

the issue with online plans is that they may not meet local building codes and, absent the architect's certification that they do, the building department won't issue the required building permit.

it follows that the way to proceed is to contact the online source and ask them to certify to your local building department that the plan as described meets the local code. if they can't or won't do this, then their plan is useless to you, except as inspiration for a local architect.

hell yea I did it when I built my barn it should not cost you top look at a pic