By strict interpretation of U.S. copyright statutes (Title 17, United States Code), that is precisely correct. Section 106 grants the copyright owner the 'exclusive right' to control the copying and reproduction of those works, "including the right to make derivative works". The object built from the plans *is* a 'derivative work'.
*HOWEVER*, only the things that are 'unique' to that particular creative effort are covered by the copyright.e.g., a plan for the construction of a "Morris-style" chair is covered by copyright. You can't photo-copy that plan and give/sell it to others. This does NOT mean that you cannot draw up a _different_ set of plans for a Morris-style chair, which is, in broad, similar to the other chair. And you _are_ free to sell/give _your_ plans to others. The 'common elements' to "Morris-style" are not protected under any copyright on a specific set of plans. Nor, except in _very_limited_ situations, are general construction techniques -- joint types, etc. What counts is the way they're put together, and the overall effect thereof.
If somebody looks at your work and says: "That's a dead ringer for a Sam Maloof design", and you _had_ copied from something Sam Maloof designed, you've got a problem. OTOH, if your work is based on your own modifications to something you saw Norm build on NYW, and you've never even _heard_ of Sam Maloof, copyright does _not_ come into play.
As I said in another post, copyright law is a *deep* legal morass. You've just touched on one of the 'stickier' issues in that morass.
The existent body of case law on copyright *does* allow for "parallel, _independent_ development" of 'similar' (or even *identical*) works.
There are cases on record where nearly identical things _have_ been found to be the result of independent development. Extant case law appears to indicate, however, that if you have _ever_ had access to, or been exposed to, the other party's 'original work', that it is essentially
*impossible* to then claim that your work _was_ 'independent development'; that, _even_if_you_weren't_consciously_aware_of_it_, that you were influenced by the other party's work, and, thus, your work is, at least to some degree, a "derivative work" of their effort',That said, what copyright protects is the "unique, creative effort" that goes into the production of an "original work". If an object has components that are -not- unique to that object/design, then those components themselves are not protected by the copyright on the object as a whole. The "great difficulty" arises in determining what parts are, and are not, actually so protected. The only *AUTHORITATIVE* determination is what the judge says, in court.
If you find the same design elements, the same proportions, the same 'styling', etc. in multiple plans, _from_multiple_sources_, it is, almost certainly, safe to copy those "common features". It's like the old term-paper joke: "if you steal from a single source, it's plagiarism, stealing from just two sources indicates laziness, but if you steal from three or more, it's research."