Where the Los Angeles and San Bernardino County Line Runs Through Claremont, Montclair and Upland

The Los Angeles and San Bernardino county line runs along the east side of Claremont, just east of the Claremont Colleges. Claremont is in Los Angeles County; Upland and Montclair, its neighbors across that line, are in San Bernardino County. Mills Avenue is Montclair's western boundary, Claremont McKenna's Roberts Campus sports bowl straddles the Claremont-Upland line, and Sixth Street becomes Arrow Route as it crosses into San Bernardino County. North of the cities, the same line runs an almost straight course into the mountains and splits the village of Mount Baldy.

This article walks the line segment by segment and explains how to confirm which side a parcel sits on. It is written for owners, buyers, attorneys and fiduciaries with property near the line.

Why is the line where it is?

It is a surveyed line, not a natural one. The Student Life, the Claremont Colleges newspaper, traced its history in April 2026. The boundary began at a black willow tree near the western bank of San Antonio Creek and was drawn as straight lines between landmarks and Mount Baldy. In the Pomona Valley it cuts across roads and residential blocks without regard to what is built on them.

That matters for an appraisal. Buyers feel a freeway, a creek channel or a ridge as a barrier between neighborhoods. They rarely feel a surveyed line. In places this one runs through the middle of the same kind of neighborhood, so two similar houses a short walk apart answer to different counties.

Montclair: the Mills Avenue segment

Montclair sits directly on the line. Its western boundary is Mills Avenue, which follows what was once the route of San Antonio Creek, the Arroyo de los Alisos of the area's earliest history. West of Mills Avenue are Claremont and Pomona in Los Angeles County; east of it is Montclair in San Bernardino County. Claremont and Upland also border Montclair on its north side.

The San Bernardino Freeway, Interstate 10, crosses the north of Montclair. North of the 10, Montclair's tracts meet the Claremont and Upland lines; south of it they continue toward Pomona and Chino. Postwar single-story tract houses sit on both sides of Mills Avenue, so look-alike houses there can fall under different assessors. The companion article on Mills Avenue as Montclair's western boundary covers that stretch in detail.

Upland: the Roberts Campus sports bowl and Arrow Route

West Upland meets Claremont along the west side of the city. Two landmarks make the line easy to picture.

• The Roberts Campus sports bowl. Claremont McKenna College's sports bowl on the Roberts Campus straddles Claremont and Upland, so a single campus sports facility sits in two cities and two counties.
• Sixth Street and Arrow Route. East of the Colleges, Sixth Street, a two-lane, shady street on the Claremont side, widens and is renamed Arrow Route once it crosses into San Bernardino County.

The Student Life describes the crossing in plain terms: cross Claremont Boulevard or Mills Avenue and the landscape shifts. The houses on either side of the Claremont-Upland line are often similar in era and size. The assessor, the recorder, the appeals board and the school district are not, and what changes at the county line sets those differences out one by one.

North of the cities: San Antonio Heights and Mount Baldy

Above Upland, San Antonio Heights is unincorporated San Bernardino County land: inside the county, outside any city. The San Bernardino County Assessor-Recorder-County Clerk assesses it as it assesses Upland, but city rules do not apply there, and its large horse-keeping lots form their own market.

Farther north, the line holds its almost straight course into the mountains and bisects the village of Mount Baldy in San Antonio Canyon. Cabins a short walk apart can sit in different counties, with different assessors and different recorders. A Mount Baldy cabin is compared with other mountain cabins, on whichever side of the line they sit, because access, snow, lot and seasonal use drive that market rather than the valley's tract sales.

How do I confirm which county my property is in?

Three checks settle it.

1. The property tax bill or assessment notice. It comes from the county where the parcel is assessed, and it carries the assessor's parcel number.
2. The assessor's parcel record. Look the parcel number up on the Los Angeles County Office of the Assessor site or the San Bernardino County Assessor-Recorder-County Clerk property information page. A parcel is enrolled in one county only.
3. The city, where there is one. Claremont, La Verne and Pomona are Los Angeles County cities. Upland, Montclair, Rancho Cucamonga and Chino are San Bernardino County cities. For unincorporated land such as San Antonio Heights or the Mount Baldy cabins, the parcel record is the answer.

What does the line mean for an appraisal?

Geographic competency, an appraiser's working knowledge of the market, the data sources and the records where a property sits, is a USPAP requirement. On this line it means knowing both counties' records and how buyers on both sides behave. Gordon Hall works from Claremont on both sides of the line, in Los Angeles County's eastern San Gabriel Valley and San Bernardino County's western Inland Empire.

A comparable sale, a recent closed sale of a similar property used as evidence of what the subject would sell for, can come from across the line. When it does, the report states why it was used and whether an adjustment, the amount a comparable's price is raised or lowered for a difference from the subject, is supported for location. Sometimes the sales show a difference and sometimes they show none; the report says which. The pillar article, what the county line does to comparable sales, explains how that testing is done, and what does not change at the county line covers the buyers, streets and market value the line leaves alone.

A few practical points follow from the map:

• A Claremont house is appraised from Claremont sales first. A sale in Upland or Montclair is used when it is genuinely better evidence, with the location question answered in writing. See the Claremont area page.
• A west Upland house is appraised from Upland sales first, and any Claremont sale it relies on is explained. See the Upland area page.
• A Montclair house near Mills Avenue is appraised from Montclair sales first, with any Claremont or Pomona sale tested for the location difference. See the Montclair area page.

The county's assessed value, the taxable value it enrolls for property tax, is not an appraisal on either side of the line, and neither is an automated online estimate, an agent's comparative market analysis or a cash offer. An appraisal is an independent opinion of market value as of a stated date, prepared by a licensed appraiser under USPAP. The full set of articles on this boundary is collected on the Two Counties, One Standard of Evidence hub.

Next step

If your property is near the line and you need an appraisal for a sale, an estate, a divorce, a trust or a tax appeal, request an appraisal with the parcel number in hand, or call (909) 324-6122. The fee is set by the scope of the assignment and quoted before work begins, and the fee does not depend on the value.

Sources

• The Student Life, City Limits: On the Los Angeles County line (April 24, 2026): https://tsl.news/city-limits-on-the-los-angeles-county-line/ (accessed October 30, 2026)
• Montclair, California (Mills Avenue as the western boundary; San Antonio Creek; Interstate 10): https://en.wikipedia.org/wiki/Montclair,_California (accessed October 30, 2026)
• Claremont, California (Los Angeles County; borders Upland and Montclair): https://en.wikipedia.org/wiki/Claremont,_California (accessed October 30, 2026)
• San Bernardino County Countywide Plan, San Antonio Heights: https://countywideplan.sbcounty.gov/community/valley-unincorporated/san-antonio-heights/ (accessed October 30, 2026)
• Los Angeles County Office of the Assessor: https://assessor.lacounty.gov/ (accessed October 30, 2026)
• San Bernardino County Assessor-Recorder-County Clerk, property information: https://arc.sbcounty.gov/property-information/ (accessed October 30, 2026)
• The Appraisal Foundation, Uniform Standards of Professional Appraisal Practice (USPAP): https://appraisalfoundation.org/imis/TAF/Standards/Appraisal_Standards/Uniform_Standards_of_Professional_Appraisal_Practice/TAF/USPAP.aspx (accessed October 30, 2026)

Gordon Hall, California Certified Residential Real Estate Appraiser, BREA No. AR003877 (lookup: https://www2.brea.ca.gov/breasearch/faces/party/view.xhtml?id=23601). Reports are prepared in conformity with the Uniform Standards of Professional Appraisal Practice (USPAP). Nothing here is legal or tax advice; admissibility is always the court's decision.