Black Diamond, Washington, population 4,781 (2019), lies within the foothills of the Cascade Mountains in southern King County, 30 miles southeast of downtown Seattle. The city is named for the Black Diamond Coal Company which owned a coal mine here as well as much of the town itself from the 1880s to the 1930s.

Black Diamond used TDR to preserve various open space resources including land surrounding Lake Sawyer.

In 2003, the city adopted a TDR ordinance, subsequently amended in 2009, aimed at protecting sending areas known as public benefit lands due to the presence of outstanding environmental, resource or recreation values as well as sites that preserve community character or provide for city facilities. The initial receiving and sending areas were designated by map. Developers of properties in receiving areas zoned Medium Density Residential (MDR8) can exceed baseline density of eight units per acre and achieve a maximum density of 12 units per acre via TDR.    

Sending areas zoned community commercial are allowed to transfer three TDRs per one-fourth acre. Sending areas in all other zones can transfer one TDR per quarter acre. If the city accepts ownership of a sending site for a park or trail, the TDRs available for transfer are multiplied by two. The TDRs can be multiplied by three for sending sites designated as treasured places, meaning a public benefit site with such environmental, cultural, aesthetic, community or strategic significance that immediate acquisition by the city is considered of utmost importance to the public welfare.

The ordinance created a TDR bank stocked with 1,000 city development rights certificates (DRCs) representing a specific number of development rights. The bank must sell DRCs for no less than fair market value as determined by an appraisal. However, the bank notifies all owners of sending area land of a proposed DRC sale and delays the sale if any of these sending area owners agree to sell their TDRs on the same terms and conditions. If no private sending site owners come forward, the bank proceeds with a sale by sealed bid.

In 2005, Black Diamond and King County, assisted by the Cascade Land Conservancy, entered into several agreements that used TDR and $1 million of King County’s Conservation Futures Tax to permanently preserve over 2,500 acres of open space and working forests. Specifically, King County allowed Black Diamond to expand its Urban Growth Area, permitting 329 acres to be annexed into Black Diamond and developed by its owners, the Plum Creek Timber Company.

In return, an easement was placed on 1,600 acres of adjacent land, known as Ravensdale Ridge, permanently creating a greenbelt northeast of Black Diamond that Plum Creek can only use as a working forest. This easement also protects more than 10 miles of trails for hiking, mountain biking, and horseback riding. Plum Creek also agreed to give King County another 645 acres of open space, including portions of the Ravensdale Creek stream corridor, which is critical to the water quality of Lake Sawyer. In addition, this action was used as a match for another $3.6 million worth of funding from the U.S. Forest Legacy Program, allowing the preservation of thousands of additional acres of working forests southeast of Black Diamond. As a bonus, the agreements created 388 acres of open space within the City including the transfer of the 165-acre Lake Sawyer Park from the County to the City and a dedication of 91 acres of land from Plum Creek to the City.