Washington case law defines a "public place" for the purposes of 66.44.270 as a place in which the general public has an unrestricted right of access or which are generally used by the public. In the case on point (State v. S.E., 90 Wash.App. 886, 954 P.2d 1338), the Washington Court of Appeals held that: “'Public place,' under statute making it unlawful for person under age 21 to be in a public place while exhibiting the effects of having consumed liquor, does not include the back patio of a residence or the common area walkway in an apartment complex."
It sounds like you may have a good defense. If you would like to discuss this matter further, please feel free to contact me.
-Luke Baumgarten, Baumgarten Law Office PLLC