Can a Washington landlord increase rent retroactively?
No. A rent increase in Washington only takes effect on or after the effective date stated in the notice, and only if the advance notice period was met (Washington HB 1217 (2024) requires 90 days advance notice for any rent increase, prohibits any increase during the first 12 months, and caps annual increases at the lower of CPI + 7% or 10% flat. Buildings under 12 years old are exempt. The rent cap framework sunsets July 1, 2040.). A landlord cannot demand back-dated rent at a higher rate for past months. Doing so makes the increase void as to the back-dated period. The tenant continues to owe the original rent for any month already in progress when the notice was served.
Source: RCW 59.18.140 (90-day advance notice for any rent increase, HB 1217)
This is an informational answer based on RCW 59.18.140 (90-day advance notice for any rent increase, HB 1217) as of early 2026. It is not legal advice. Housing law changes year to year and local ordinances (especially in rent-controlled or rent-stabilized cities) can override or add to state law. For contested cases, consult a Washington-licensed attorney.