Social Security Questions? Ask a Social Security Expert
Social Security is a federal program so any lawyer can handle that matter. No need to be a Washington State lawyer.
Since Washington has its own disability program, the State of Washington Disability Determination Services determines eligibility for Social Security Disability and Supplemental Security Income, so if you are applying through that department you would want to get a Washington State attorney as the state may have specific rules and procedures unique to it.
States that do not offer a disability program apply directly to the SSA through one of its offices.
I hope this answers your question. Please let me know if I can clarify anything or answer any other questions.