Mercer Island tree permits, retention, and replacement rules

Start with size, status, and location

For tree removal that is not related to development, the city's current application guide requires a permit when a tree is at least 10 inches in diameter, is exceptional, or is in a critical area. Trees under 10 inches generally do not need a permit only when they are not exceptional, not in a critical area, and not a required replacement tree.

Use the city's private-property tree permit page and interactive tree map to screen the site before assuming a smaller tree is exempt.

Exceptional trees are not defined by 36 inches alone

An exceptional tree may qualify because of its size, species, condition, age, cultural importance, or contribution to a grove. The city guide treats any tree over 36 inches as exceptional, but it also publishes much smaller species-specific thresholds, including six inches for several native species.

• Identify the species before applying the threshold table.
• A grove can create exceptional status even when an individual trunk is below 36 inches.
• Map exceptional trees early because they receive priority in development retention decisions.

Development looks back five years

Tree removal associated with construction is reviewed with the development permit. If development is planned within five years, Mercer Island's guide says at least 30 percent of the trees that are 10 inches or larger and exceptional trees must be retained over a rolling five-year period. Removing trees before design can therefore delay or constrain a later project.

• Have a qualified arborist inventory trees before removal or concept design.
• Retain exceptional trees, large trees, and groves where the development standards prioritize them.
• Show tree protection, grading, access, utilities, and the building footprint together rather than treating the arborist plan as a late overlay.

Replacement increases with diameter

The non-development guide calls for two replacements for a removed tree from 10 up to 24 inches, three from 24 up to 36 inches, and six for a tree over 36 inches or any exceptional tree. A qualifying tree under 10 inches has a one-for-one ratio, although no replacement is needed when it is under 10 inches, not exceptional, and not a prior replacement tree.

• Replacement conifers are generally at least six feet tall; deciduous replacements generally have a 1.5-inch basal diameter.
• Primary native Pacific Northwest species are preferred.
• The city arborist may authorize fee-in-lieu only when there is not enough planting area on site or in the adjacent right-of-way.

Critical areas receive extra review

Critical areas include steep slopes, wetlands and buffers, watercourses and buffers, and bald-eagle nest buffers. Removal there can require additional plan review and a hold-harmless or indemnification agreement. Use the city map as a screening tool, then confirm the actual feature and buffer when a project depends on the answer.

Read the city's tree-removal guidance for critical areas before pruning or removing a tree in a mapped or suspected critical area.

Pruning and hazards still have limits

Outside critical areas, pruning up to 25 percent of total leaf area may be performed without a tree permit when it uses accepted crown-thinning, cleaning, windowing, or raising methods. Topping or work likely to kill or significantly damage the tree is prohibited, and trimming right-of-way trees is not allowed without city authorization.

• An imminent hazard can be removed immediately after notifying the city and documenting the condition.
• Apply for the after-the-fact hazard permit within 14 days and include photographs or a qualified-arborist report.
• Hazard removal still carries replacement requirements.

Our take

Do not clear trees to make a concept plan easier. On Mercer Island, that can reduce the rolling retention count, erase the best trees for the required 30 percent, and complicate critical-area review. Inventory first, then fit the house, access, and utilities around a defensible retention plan.