Arborist tree inventory and protection plan

The tree code in one paragraph

Since 2023, Seattle has had a four-tier tree protection system (SMC 25.11), tightened again in late 2025. Tier 1 (heritage) trees can almost never come down. Tier 2 trees — diameter at standard height (DSH, 4.5 ft) of 24 inches or more — generally can't be removed unless they're hazardous or needed for "development capacity." Tier 3 (12–24") and Tier 4 (6–12") can come out but trigger replacement requirements. Anything under 6" DSH is unregulated.

A Tier 2 tree's drip line is off-limits to construction. A 30-inch Douglas fir at the back of the lot can shrink your buildable footprint by hundreds of square feet.

What the arborist produces

• Tree inventory — every regulated tree on the lot, plus any off-lot Tier 1 or Tier 2 tree whose drip line crosses onto the lot. Species, DSH, drip line, condition.
• Tree protection plan — fencing, no-go zones, monitoring schedule, air-spading requirements if the foundation comes near a critical root zone.

Both get overlaid on the survey and submitted with the construction permit.

Hiring criteria

• ISA Certified Arborist (look them up at treesaregood.org).
• Firm on the Seattle Tree Service Provider Registry with $1M general liability — required since 2022.
• TRAQ-qualified if any hazard work might be needed.

Cost

Inventory + protection plan: $300 – $800 for a typical Seattle SF lot. Construction monitoring (visits during excavation and framing near retained trees) is extra and can run into the thousands.

Why sequence matters

If the arborist comes in after the architect's schematic, you risk finding out that a Tier 2 tree's drip line clips the proposed cottage corner. That's a redesign — not just a footprint shift, but a re-coordination across structural, civil, and energy. The arborist's $500 happens before the architect's $20K, not after.