RCW 58.18.010
Assessor's plat—Requisites, filing, index, etc.—When official plat.
In any county where an assessor has and maintains an adequate set of maps drawn from surveys at a scale of not less than two hundred feet to the inch, the assessor may with the permission of the county commissioners, file an assessor's plat of the area, which when filed shall become the official plat for all legal purposes, provided:
(1) The plat is filed in the offices of the county auditor and the county assessor, together with a list of the existing legal descriptions and a list of the new legal descriptions as assigned by the county assessor;
(2) The recorded plat is drawn in such a manner that a ready reference can be made to the legal description in existence prior to the time of the filing of the assessor's plat and in conformance with existing statutes;
(3) The first year the tax roll and tax statement shall contain the prior legal description and the new legal description as assigned and shown on the assessor's plat with a notation that this legal description shall be used for all purposes;
(4) The county assessor shall maintain an index for reference to the prior and the existing legal descriptions of the parcels contained in the assessor's plats;
(5) Each dedicated plat after June 7, 1961, shall be submitted to the county assessor of the county wherein the plat is located, for the sole purpose of assignment of parcel, tract, block and or lot numbers and the county auditor shall not accept any such plat for filing unless the said plat carries a signed affidavit from the assessor to this effect, and a statement to the effect that the name of the plat shall be number . . . . in the county of . . . . . .
[ 1961 c 262 § 1.]