The new regulations are designed to:

• Retain as many of Seattle’s really significant trees, and retain (and expand) as much of Seattle’s urban forest on land undergoing development, as is compatible with other objectives,
• Allow development to occur, including affordable housing, and
• Minimize permitting delays and additional costs.

The tree protection guidelines include the following requirements, which are summarized in more detail, in Client Assistance Memo #242, "Tree Protection Regulations in Seattle" (400KB PDF):

• Removal of a tree over six inches in diameter is prohibited on undeveloped land unless the tree is deemed hazardous or is being removed in conjunction with development. Also, removal of an exceptional tree is prohibited on lots containing a single family house in a zone other than single family.
• Buildings may need to be designed, and some development standards modified, to avoid exceptional trees.
• All trees over six inches in diameter must be shown on site plans for new construction (not additions or remodels) in residential small lot, single family, low rise, midrise and commercial zones, and for short plats or subdivisions, and projects undergoing Design Review.
• For trees that are two feet in diameter or greater, some development standards may be modified at the option of the applicant to protect all trees.

Standards and procedures for determining whether a tree is exceptional (for both native and non-native species) are outlined in a new Director’s Rule, DR 6-2001 (101KB PDF).  This rule is titled "Clarification of State Environmental Policy Act (SEPA) Plants and Animals Policy Concerning Outstanding Trees; and Designation of Exceptional Trees under the Tree Protection Chapter (25.11) of the Seattle Municipal Code." 

A handy illustrated guide to trees, called "Common Seattle Trees" (373KB PDF), describes trees that may qualify for protection.  For each species, drawings of leaf shape and overall form of the tree are shown, in addition to comments on the fruit, flower, bark, cones (when applicable), and tree tip.

The adopted regulations match those outlined in the May 2001 issue of DCLUINFO (1MB PDF), with one change: "exceptional trees" were previously referred to "heritage trees."