My 17 year old moved in with her dad. Her dad asked me how much I am going to pay him to supprt her? I told him I never asked him for support. Can he do this for the last 7 months till she is 18? Will a court take everything into account, like the past?

Washington uses a very specific form to determine the amount of child support due. No, the past does not count.

If there is an order that he should pay you it is in effect until it is changed by the Court.

Is there an order awarding you custody? If there is, she can't just move in with dad. He has to file for a modification of the custody order. As part of that he can also apply for support.

Until a court orders support you don't have to pay anything.

Did you ever go get a court order for support ? If so he owes you all of that back support.

If you never went to court to get support, then no, he does not owe you and no they will not take that into account

And yes he can file in court for support if he wants to and you will have to pay according to their rules.