What's an addendum in a rental agreement?

Speaking generally, if your agreement is a lease, neither the landlord nor tenant can amend it unless both parties agree (the same can be said for a completely new agreement). If it is a month to month agreement, if you don't sign, the landlord may be able to give you notice to vacate. What I would do is ask for a couple of days for review. Put your current agreement down with the proposed new agreeement down, side by side, and go through them together, paragraph by paragraph. At the same time, make notes about anything in the new agreement you don't like, and attempt to negotiate the terms with your landlord before signing. Please note that the WA State Residential Landlord Tenant law was modified in several ways by the WA State Legislature, effective a few months ago - this may be why your landlord is doing this. For example, a landlord now has 30 days to return a security deposit (or explain why he/she isn't); it was 21 before that, and 14 before that). To review the Landlord/Tenant Law at the same time, follow this link: https://apps.leg.wa.gov/rcw/default.aspx?cite=59.18 If you would like to hire me to help, I am not available until October 15th, I'm afraid - I'm on vacation at Yosemite.

It is customary for the landlord to keep the original. WA State Law requires the landlord to give the tenant a copy: https://apps.leg.wa.gov/rcw/default.aspx?cite=59.18.065

A Washington State residential landlord/tenant template may or may not be reliable depending on who drafted it or provided it, whether or not it applies to a particular situation, and whether it is up to date (in 2023, WA State made several changes to the law). You can review the state law here: https://apps.leg.wa.gov/rcw/default.aspx?cite=59.18 However, some cities/counties have additional requirements and/or provisions that also must be followed. It would have been a good idea to have an attorney review the document BEFORE you signed it.

Residential month to month rental agreements and/or residential lease agreements often include terms regarding whether or not an agreement automatically renews. However, the Washington State Residential Landlord Tenant Law and/or ordinances in certain WA cities sometimes may provide the answer to your question. https://apps.leg.wa.gov/rcw/default.aspx?cite=59.18

When your lease expires, then it is over unless you sign a new lease or fail to give your landlord a 30-day notice. Send your notice and move out when before the lease expires, which could be the last day of the lease.