Tree removal and trimming rules for unincorporated San Mateo County, CA. Check your address to confirm you are outside city limits and see whether your tree needs a county permit.
A permit is needed to trim or cut down a live Significant Tree (38"+ in circumference, about 12.1" diameter, at 4.5 ft; 19"+ in the Emerald Lake Hills Design Review district) or a Heritage Tree (all Santa Cruz cypress and Oregon white oak, plus other trees by size and location).
How to apply: Apply to San Mateo County Planning & Building for a tree pruning or removal permit (arborist report, photos and site posting may be required).
How to measure: most Bay Area cities measure trunk diameter (or circumference) at 4.5 ft (54") above natural grade, but some measure elsewhere: Berkeley at 4 ft, Redwood City at the widest point between 6" and 36". Where a city sets its limit by circumference, we show the diameter (circumference ÷ 3.14).
Official sources: smcgov.org/planning/how-do-i-trim-or-cut-down-tree · smcgov.org/planning/tree-regulations