Are you talking about building to 'exact' specifications?
I stated I'd like to build an LX4/LX6. In real life these cars are comparable to Lotus 7's, Caterham's etc. Or the popular choice for builders; a 'locost'. This doesn't violate copyrights, yet I would still consider when I do end up building mine that it will be like an LX4/LX6 depending on how I do the build.
Eitherway, it wouldn't violate any copyrights or laws to build something to the specifications of a RAC, MRT, BF1 etc - it would only do so if you tried commercially selling it to race teams or the public for that matter. For personal use there isn't anything anyone can do with you buying all the parts and following a pattern/specification etc... It only fails legally when trying to sell the product or trying to patent the idea as your own.