If the house is inside the Seattle city limits, you can terminate a month-to-month tenancy only for just cause. If you are truly moving back into the house as your primary residence, you have just cause to terminate the tenancy, and you should definitely state that reason in your notice of termination. You must actually accomplish the move. If you say you are moving and then don't do it, you will face substantial penalties. The "Just Cause" ordinance is Seattle Municipal Code section 22.206.160C. Be aware that giving a notice to terminate the tenancy must be done in exactly the right way according RCW 59.12.040, by personal delivery of a copy to each of the adult tenants at least 20 days ahead of the next rent due date. If not all the tenants are available to receive the delivery, or if one or all refuse to take it, you must post the notice and mail a follow-up copy by ordinary first class mail, for each tenant, and if the post-and-mail method is used, the advance notice must be at least 21 days, not 20. Don't try to short-cut this process, and if you are not sure about it, call an experienced landlord-tenant lawyer. The City of Seattle Department of Development and Design has some good materials online. Links to these references are posted below this answer. For the Just-Cause ordinance, you have to scroll down pretty far to see the right paragraph.
This answer is intended as a courtesy only, and does not constitute an attorney-client relationship between the attorney and the questioner.
If you want to set the date to terminate tenancy out further than 20 (or 21) days from the date you give valid notice, that is not a problem. But make the termination date the last day of the month at midnight, and provide a sufficiently thorough explanation of the cause. Also, beware of retaliation claims that may arise.
I hope this helps answer your question. Talk with a good landlord-tenant attorney about your specific facts to find out your rights.
This posting is for informational purposes only. It is not legal advice and does not establish an attorney-client relationship. <a title="Attorney Ryan A. Jacobsen" href="http://jacobsenlawoffice.com/about/">Ryan A. Jacobsen</a> | <a title="People | Property | Business" href="http://jacobsenlawoffice.com/">Jacobsen Law Office, P.S.</a><br /><b> <a title="People | Property | Business" href="http://jacobsenlawoffice.com/people/">People</a> | <a title="People | Property | Business" href="http://jacobsenlawoffice.com/property/">Property</a> | <a title="People | Property | Business" href="http://jacobsenlawoffice.com/business/">Business</a></b>