Generally, the board will review only documents and exhibits taken from the record developed by the city, county, or state in taking the action that is the subject of review by the board and attached to the briefs of a party. A party by motion may request that the board allow the record to be supplemented with additional evidence.

(1) A motion to supplement the record shall be filed by the deadline established in the prehearing order, shall attach a copy of the document, and shall state the reasons why such evidence would be necessary or of substantial assistance to the board in reaching its decision, as specified in RCW 36.70A.290(4). The board may allow a later motion for supplementation on rebuttal or for other good cause shown.

(2) Evidence arising subsequent to adoption of the challenged legislation is rarely allowed except when supported by a motion to supplement showing the necessity of such evidence to the board's decision concerning invalidity.

(3) Exhibits attached to motions to supplement shall be cross-referenced in the briefs for the hearing on the merits, unless the presiding officer, in the order on motion to supplement, requires copies of supplemental exhibits to be attached also to the hearing on the merits brief.