I'm not a New Hampshire attorney, but in most states, the "posting" requirement only applies to undeveloped land.  A vacant building isn't an invitation to the public, it's private property.If you've been arrested for trespassing in a vacant building, there may be other facts that affect your case.  For reliable advice, based on all the circumstances, you should talk to a lawyer, and one place to find counsel is our website, http://attorneypages.com