Tree Protection Zones (TPZ) Explained — What Homeowners & Contractors Need to Know in 2025

A practical guide to understanding, planning, and complying with tree protection requirements in Seattle, Bellevue, and across Western Washington in 2025.

🌲 What Is a Tree Protection Zone?

A Tree Protection Zone (TPZ) is a designated area around a tree’s trunk that must remain undisturbed during construction or site work. Cities require TPZs to:

• Protect the critical root zone (CRZ) where trees absorb water and nutrients.
• Prevent soil compaction, grade changes, and root cutting that destabilize or kill trees.
• Keep trees healthy long-term, reducing hazard risks and future removal costs.

Think of it as the “Do Not Disturb” zone that keeps a tree alive and stable while construction happens nearby.

📏 How Cities Calculate TPZ Size

Most Puget Sound municipalities base TPZs on the tree’s Diameter at Standard Height (DSH), measured 4.5 feet above the ground:

• Seattle & Bellevue: Typically require a TPZ radius of 1 foot per inch of trunk diameter (e.g., a 20″ tree = 20-foot radius).
• Renton & Issaquah: Use similar formulas but may allow modified shapes when space is limited.
• Mercer Island: Comparable approach but requires documentation from an ISA Certified Arborist if reductions are requested.

🧮 Example: A 24″ DSH bigleaf maple would often need a 24-foot protection radius — nearly 50 feet across.

🏗️ Why TPZ Compliance Matters

• Permit Approval: Incorrect or missing TPZ plans can delay or deny your building permit.
• Fines & Stop-Work Orders: Cities can halt construction or issue penalties if TPZ fencing is absent or disturbed.
• Tree Safety & Liability: Damaged roots can cause tree failure, leading to property damage or injury.
• Property Value: Preserving mature trees boosts curb appeal and long-term property value.

🛠️ How an ISA Certified Arborist Helps

TPZ rules can be complex — especially on tight lots or remodels.
Our ISA Certified Arborists:

• Prepare Tree Protection Plans (TPPs) that meet ANSI A300 standards and local city codes.
• Accurately map and modify TPZs when full-radius protection isn’t practical (e.g., property lines, driveways, utilities).
• Provide documentation city reviewers require for permits and inspections.
• Monitor TPZ installation to prevent costly mistakes or fines.

 Can TPZ Size Be Reduced?

Yes — but only with proper documentation. Cities like Seattle, Bellevue, and Issaquah allow modified TPZ shapes or reduced radii when:

• Existing impervious surfaces (driveways, sidewalks) are within the zone.
• Property boundaries or utilities make a full-radius impossible.
• A Qualified Tree Risk Assessor (TRAQ) confirms a smaller zone won’t destabilize the tree.

⚖️ Always have any reduction or modification prepared and signed by an ISA Certified Arborist — it’s the key to permit approval.

🧪 Common TPZ Mistakes to Avoid

• 🚧 Skipping a professional plan: DIY fencing rarely meets city standards.
• 🏗️ Staging equipment inside the TPZ: Even brief compaction damages roots.
• 🪚 Cutting roots without approval: Can destabilize trees and violate your permit.
• 🧱 Changing grade within the TPZ: Adding or removing soil suffocates roots.

🌱 Beyond Construction — TPZ & Tree Health

Even outside of permitting, protecting the root zone keeps mature trees healthy and stable. Our Plant Health Care (PHC) services — soil aeration, mycorrhizal inoculation, balanced fertilization, and pest/disease management — help maintain strong root systems and extend tree life.

✅ Work With Certified Arborists for TPZ Compliance

Every city handles tree protection differently, but one rule is universal:
Partnering with an ISA Certified Arborist saves time, avoids fines, and protects your trees and property value.

Sound Tree Care LLC provides:

• ISA Certified Arborist Reports & TRAQ Risk Assessments
• Tree Protection Plans & TPZ Design/Monitoring
• Permit Navigation for Seattle, Bellevue, Renton, Mercer Island, Issaquah & Beyond
• Plant Health Care & Tree Preservation