Right, the governing body can make an exception for a particular private road. And in that case, you would be able to apply for a building permit. There are also provisions for exceptions in RSA 674:41 that would cause excessive hardship or practical difficulties. So it seems that the law has been made so that a local governing body has the discretion to made a pragmatic decision. However, this all assumes that the local governing body would indeed grant such an exception and that the land owner believes it is worth to effort. In the case of your hypothetical farmer, it is probably worth the effort. However, if you have a 2 acre lot and want to put a farm in a box on it, you might be happy to simply avoid dealing with the governing body and just plant your farm in a box on your lot and get to work.