Short version, all Washington state courts are required by law to provide either lock boxes OR an officer to take possession of your firearm for the duration of your visit to the court house. Grape sited the law.
"For purposes of this subsection (1)(b), "weapon" means any firearm, explosive as defined in RCW 70.74.010, or any weapon of the kind usually known as slung shot, sand club, or metal knuckles, or any knife, dagger, dirk, or other similar weapon that is capable of causing death or bodily injury and is commonly used with the intent to cause death or bodily injury.
In addition, the local legislative authority shall provide either a stationary locked box sufficient in size for pistols and key to a weapon owner for weapon storage, or shall designate an official to receive weapons for safekeeping, during the owner's visit to restricted areas of the building. The locked box or designated official shall be located within the same building used in connection with court proceedings. The local legislative authority shall be liable for any negligence causing damage to or loss of a weapon either placed in a locked box or left with an official during the owner's visit to restricted areas of the building."
There are a few courts that will try to require that someone "checking in" a firearm also provide identification before they will allow you to "check in" your firearm. This is NOT a requirement of the law and thus is unlawful. My personal suggestion is to have a copy of RCW 9.41.300 printed out and and present it to any officer that tries to require identification. Be prepared to ask officers questions such as "Am I required to have identification on my person while walking down the street?" "Am I required to have a CPL on my person while lawfully open carrying in Washington state?" If the response to these questions is "No." Then follow up with "Then why am I required to provide identification to check in my firearm when entering the courthouse, when I am not required to have identification while walking down the street or have a CPL on my person while lawfully open carrying?"
Also, be prepared to record (at the very least audio) the interaction with the officer(s) (and you do not need to inform the officer(s) that you are recording as federal courts have ruled that any and all officers have no reasonable expectation of privacy during the commission of their duties as public servants).
Mind you, this does NOT apply to federal courts within Washington state.