San Mateo County, CA Noise Ordinances: Decibel Limits (2026)

Section 4.88.330 makes it unlawful for any Person to create noise, or to allow it on property the Person owns, leases, occupies or otherwise controls, that pushes the exterior noise level above Table I at a single or multiple family residence, school, hospital, church or public library. The source must sit in the unincorporated area, but the receiving property can be in an incorporated city or in unincorporated territory, so a loud source on county land is measured against a neighbor across a city line.

Table I has five categories, each tied to the cumulative minutes in any one hour. Category 1 is 30 minutes at 55 dBA daytime and 50 dBA nighttime. Category 2 is 15 minutes at 60 and 55. Category 3 is 5 minutes at 65 and 60. Category 4 is 1 minute at 70 and 65. Category 5 is 0 minutes at 75 and 70. Daytime runs 7 A.M. to 10 P.M. and nighttime runs 10 P.M. to 7 A.M.

Three adjustments sit under the table. When measured background noise already exceeds a category standard, the standard is adjusted in five (5) dBA increments to encompass the background level. Every standard is reduced by 5 dBA for simple tone noises, consisting primarily of speech or music, or for recurring or intermittent impulsive noises. A continuous source that cannot reasonably be stopped long enough to read the background is compared directly to Table I while it runs.

Measurement follows section 4.88.320. The inspector uses a sound level meter on the "A" weighted network at slow response (fast response for impulsive sounds), calibrates with an acoustic calibrator immediately before recording, and fits a windscreen. No outdoor reading is taken during precipitation or when wind exceeds 12 miles per hour. Exterior readings are taken within 50 feet of the affected residence, school, hospital, church or public library but never beyond the property line, with the microphone four to five feet above the ground and ten feet or more from a reflective surface where practical.

Section 4.88.360 exempts several activities from the chapter, including school events, public parks and playgrounds, emergency work, and demolition, construction, repair, remodeling or grading that stays out of 6:00 P.M. to 7:00 A.M. on weekdays, 5:00 P.M. to 9:00 A.M. on Saturdays, and all day Sunday, Thanksgiving and Christmas. Residential property maintenance is exempt between 7:00 A.M. and 8:00 P.M.