There are two different issues here:1) Can you cancel because your neighbor won't allow access: no--not unless the contract you signed said that under this sort of situation, you could cancel and get your money back. There is no general right to cancel and get your money back because of an impediment or problem which was not under the contractor's control (i.e. not something they were responsible for).2) Do you have to pay if the "engineering was a disaster"--maybe not. It depends on how bad it was, and whether they corrected or are in the process of correcting the problem. A breach of contract, which includes not correctly or competently doing the work for which they were hired, can give you the right to treat the contract as terminated and not pay amounts due under it, but only if a) the error or breach was "material" or important/significant, and b) they did not or are not in the process of fixing the problem, since if the problem is "cured," it is not a breach of contract.Note that even if you believe the drawing, etc. problem was so bad as to justify not paying, if they disagree (as they presumably will), you can expect to get sued for the money; if sued, you'll likely have to hire a lawyer, spending money, which amount will be out of pocket (you won't get  it back), will have to spend time on the case, and could lose (you can never guaranty a win; even good cases lose sometimes). Depending on how much is at stake, it may or may not be worth litigation.