When the final counter offer is accepted by all parties, and the last party to sign it officially delivers it to the other side’s agent, you have a binding agreement! Note if you are the last party to sign the counter offer (meaning you are accepting the sellers counter offer as it is) and you sign it on Thursday night, but your agent doesn’t email, fax or hand deliver it until Friday, the binding agreement date is Friday, not when you signed it. The offer has to have been received by the other party to have binding agreement. This is documented with either an email sent receipt or a fax confirmation.
Congratulations! You are now “Under Contract!” This is when the clock starts ticking on all of your time lines so it is very important to do all of the things your agent instructs you to do in a very timely manner.