In Washington there is sometimes confusion about how many days notice a landlord is required to give a commercial tenant to end a month-to-month tenancy. Some say thirty days is required, while only twenty days is required for a residential tenant. Others say only twenty days is required for either a commercial tenant or a residential tenant.
The answer requires a short history lesson.
More than a century ago, when Washington was still a territory, a law was passed allowing either party to terminate a month-to-month tenancy with thirty days notice. The applied to all tenancies, whether commercial or residential. This law is still in effect.
A few decades later the legislature passed the unlawful detainer statute, still in effect and commonly used today. This law applies to all tenancies, whether commercial or residential. It only requires twenty days notice to terminate a month-to-month tenancy.
About a century later the Residential Landlord-Tenant Act (RLTA) went into effect in 1973. The older unlawful detainer statutes still apply, except to the extent superseded by the Residential Landlord-Tenant Act. The RLTA has a provision that allows either party to terminate a month-to-month tenancy with twenty days notice.
The answer is that as the unlawful detainer statute only requires twenty days notice, only twenty days notice is required for either a commercial tenancy or a residential tenancy.
The parties should check the lease (even if the tenancy is now month-to-month). Some leases require more notice than the minimum required by statute. Also, residential landlords in Seattle must have just cause to evict, even if the tenant is now month-to-month
 RCW 59.04.020.
 RCW Chapter 59.12.
 RCW 59.12.030(2).
 RCW Chapter 59.18.
 See for example, RCW 59.18.420;