Wash. Rev. Code § 59.18 § 59.18.740

(1) Except as provided under subsections (3) through (5) of this section, a landlord may not prohibit or restrict a tenant from installing a portable cooling device of the tenant's choosing.
(2) A landlord may not require a fee for the use, inspection, or installation of a portable cooling device. Nothing in this section shall preclude a landlord from charging for the rental of a portable cooling device rented by a tenant from the landlord, nor for any damages arising out of a tenant's use of a portable cooling device.
(3) A landlord may prohibit or restrict a tenant from installing a portable cooling device of any type if the dwelling already has a permanently installed and fully operational heat pump, or if installation of the device would:
(a) Violate state or local building codes, state law, or federal law;
(b) Violate the device manufacturer's written safety guidelines for the device;
(c) Cause unreasonable damage to the premises or render the premises uninhabitable; or
(d) Require an electrical supply to power the device that cannot be accommodated by the existing power service to the building, dwelling unit, or circuit.
(4)(a) A landlord may prohibit or restrict a tenant from installing a window-mounted portable cooling device if:
(i) The window is a necessary egress from the dwelling unit and the device would interfere with its use as egress;
(ii) The device would interfere with the tenant's ability to lock a window that is accessible from outside;
(iii) The device requires the excessive use of brackets or other hardware that would damage or void the warranty of the window or frame, puncture the exterior wall of the building, or otherwise cause significant damage;
(iv) The device cannot be secured in a way that prevents it from falling out of the window; or
(v) The landlord's insurance policy for the dwelling occupied by the tenant expressly restricts or prohibits the use of window-mounted portable cooling devices, and the landlord has provided written evidence of insurance policy restrictions or prohibitions to the tenant.
(b) As a condition of installing a portable cooling device, a landlord may require that the device be adequately drained to prevent damage to the dwelling unit or building.
(5) To verify compliance with the installation limitations allowed under this section, a landlord may also require that a window-mounted portable cooling device be subject to one or more of the following:
(a) Inspection after installation; or
(b) Servicing after installation.
(6) A tenant must provide at least two days' notice to a landlord prior to the installation of a window-mounted portable cooling device.
(7) A landlord is immune from liability for any claim for damages, injury, or death caused by a portable cooling device installed by the tenant.
(8) If a landlord restricts or prohibits installation of window-mounted portable cooling devices, the landlord must notify tenants in their leases of their rights, responsibilities, and any restrictions or prohibitions related to installation of window-mounted portable cooling devices as specified in subsection (4)(a)(i) through (v) of this section.
(9) Nothing under this section shall be construed to limit the responsibilities of landlords to provide reasonable accommodations under existing state and federal law.
(10) A landlord is not responsible for any interruption in electrical service resulting from the installation of a portable cooling device that is not caused by the landlord, including interruptions caused by an electrical supply's inability to accommodate use of a portable cooling device.
(11) Nothing in this section shall preclude a landlord from retaining a portion of a security deposit for a lawful purpose under RCW 59.18.280 .
(12) For the purposes of this section:
(a) "Portable cooling device" means an air conditioner or portable heat pump, including a device mounted in a window or designed to sit on the floor, but does not include devices whose installation or use require excessive alteration to the dwelling unit including, but not limited to, the excessive use of brackets or other hardware under subsection (4)(a)(iii) of this section.
(b) "Saddle-mounted portable cooling device" means a portable cooling device that hangs over the windowsill with interior and exterior components rather than sitting on top of it, such that the window view is not fully obstructed and the device will not fall when the window is opened, and includes u-shaped portable cooling devices.
(c) "Window-mounted portable cooling device" means a portable cooling device that is installed to sit in the window but does not include a saddle-mounted portable cooling device, a u-shaped portable cooling device, or a floor-mounted device that is connected to a window for air exchange or ventilation.