Damages for willful infringement, where you knwoiungly copy a copyrighted work, can result in damages of up to $150,000 per work. I doubt a situation like this would have that result, but that's the potential risk here.
Maybe the 1st builder would license their plans to you, so you could use them for your house but the builder would still own their copyright.
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Just to clarify, $150,000 is the maximum statutory damage award for infringement of a work with a registered copyright (they could also get their attorneys' fees). The minimum is $750. Most likely, the plan is not registered, although it may be difficult to determine that, in which case they could only recover their actual damages.
Copying the original and making a few changes would not avoid copyright infringement.
Although I agree with the answers in terms of the illegality of copying the plans (even with "minimal" changes), this does not necessarily foreclose any "plans" to build a house of the same general design as the one you had the first builder draft architectural drawings for. It's the drawings that are protected by copyright. While I'm not aware of any cases to cite, you can walk into a model home, fall in love with it and have a similar home built as long as no plans are copied. How many home designs can there be? Taken to the extreme, a strict application of copyright protection to houses can freeze the home building industry in its tracks. For this reason, a "basic" architectural drawing would be given a narrow scope of protection on the originality scale.
As the other attorneys indicate, there's a IP issue here. So why don't you go about it a different way that builders understand. Tell the first builder the truth - you have another (presumably) better bid and can they match it? Try to get things back on a business level. Also, I don't understand why the second builder would take 2 to 3 weeks to draft plans. If that's any indication of how much longer they would take with your project, as opposed to the first builder, that's something to consider as well. Good luck.
Kevin B. Murphy, B.S., M.B.A., J.D.
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