Its kinda of a mute discussion as I'm more than willing to pay the original designer for the plan. As I noted, I have already tried to contact them for modifications.
While I am not a copy-write guru at all, I do work for a large software developer and deal with software patents all the time. So there is some resemblance.
I suspect there has probably never been a successful legation for this type of usage. Imagine the following scenario.
I find a plan online and blatantly copy it to build a house. The original designer just so happens to be vacationing in my state and drives by my house. He says "He that looks like my house".
He hires a lawyer and he will need to hire an architect. To proceed with the case he has to get a judgment for discovery and warrant so the interior of my house can be evaluated by an expert witness.
It now goes to court and he has to convnce a jury his design is orginal, despite the volumes of prior art for nearly identical designs that pre-date his. Now lets assume after all that, the Judge/Jury finds in his favor. His damages are limited to the cost he was selling his plan for online....a measly couple hundred bucks.
Its simply not worth the cost nor is it likely someone will random pick out a house they designed from the millions of homes across America.
Remember, this is not copying for the sake of mass distribution or sale. Damages are limited to the lost sale.
The law is really only useful when someone designs a plan that is not intended to be publicly for sale, or it designed to be an "original". In this case, the "loss" would be a bit more in tangible as an original creation is no longer original. But when a plan is available for public sale, by its very nature its designed to be used to built and modified (all homes get modified at a minimum by onsite contractors).
Would I suggest someone copy a design thats truly original like a world recognizable museum? Or copy a design and then try and sell the design online to hundreds of people as your own? No.
Nor would I suggest not buying form the original if possible and it fits ones needs. But I suspect that building a plan for an owner/occupier from rough pictures online is quite safe.