This classification is reserved for land that is designated by an official comprehensive land use plan to be retained in its natural state in order to enhance, protect or preserve natural areas, such as wetlands conservation, wildlife and nature sanctuaries, historic sites, and protection of streams and water supplies (RCW 84.30.020). The taxable value is determined according to a Public Benefit Rating System, with a minimum of three (3) points required, where three points would equal a 30% reduction to market value, four points would be a 40% reduction, and so forth.

Applications and fees are submitted to the Cowlitz County Department of Building & Planning.

Once land is classified, the Assessor's Office maintains the files for all property in the Open Space program, including maintaining a dual tax roll to track the Current Use value (taxable) and the true and fair value (market). The Assessor is also responsible for conducting periodic reviews including on-site inspections to make sure the land is still being used for the purpose for which it was granted classification. If land is no longer being maintained according to the approved Open Space agreement, it will be removed from classification and an Additional Tax will be assessed, including interest and a 20% penalty.