Where the growth came from
Countywide roll growth has three sources: new construction adding property that was not on the roll, changes of ownership resetting base-year values, and revaluation of property that was already there.
Contra Costa's 2026-27 numbers let you narrow that down. The roll grew 3.48 percent. The parcel count grew by 1,169 out of 383,191 — roughly three-tenths of one percent.
New parcels are not carrying this. Whatever the roll gained, it gained overwhelmingly from property that was already on it: reassessment on sale, the Proposition 13 inflation factor, new construction on existing parcels, and the expiry of prior decline-in-value reductions.
That last item is the one owners forget, and in a county recovering from a soft patch it can be the largest single movement on an individual parcel.
When a Prop 8 reduction comes off
Proposition 8 requires the assessor to enroll the lower of the factored Proposition 13 base-year value or current market value on the lien date. A reduction granted under it is explicitly temporary and reviewed every year.
When the market recovers, the assessor restores value — and the restoration is not limited to 2 percent. The 2 percent cap governs the growth of the factored base-year value, not the pace at which a temporarily reduced assessment climbs back toward it.
So an owner who received a decline-in-value reduction in a weaker year can see a large percentage increase in a recovery year and reasonably think a mistake was made. Usually none was. The assessment is returning toward the factored base, and the ceiling on it is that base rather than last year's reduced figure.
The right question is not "why did this rise so much" but "is the restored figure at or below both the factored base and current market value on January 1?" If the assessor overshot market value on the way back up, that is a live appeal. The Proposition 8 definition sets out the mechanism.
What to check on a Contra Costa parcel
Three things, in order:
- Whether your parcel carried a Prop 8 reduction in a prior year, and what the factored base-year value is. Those two numbers frame everything else.
- Whether the enrolled value exceeds market value on January 1, 2026. If it does, the statute directs the lower figure regardless of what the base says.
- Whether anything reset the base. A change in ownership or new construction creates a new base-year value for the affected portion, and that is a different argument with a different valuation date.
For income property, the supporting documents are the ordinary ones: occupancy across the assessment year, rent achieved net of concessions, real downtime, and competing sublease space. Contra Costa's suburban office and flex product has been repricing, and where documented results do not support the enrolled value, that gap is the case.
Dates, and the evidence window
The regular assessment appeal period is July 2 through November 30, 2026, filed with the Contra Costa County Assessment Appeals Board / Clerk of the Board at 1025 Escobar Street, Martinez. November 30, 2026 is a Monday, so no weekend extension applies.
Contra Costa is a November 30 county because its assessor does not send value notices to all secured-roll assessees by August 1 — the test that puts Alameda and Santa Clara on a September 15 deadline instead. If you hold property across the bay, you are working to two different dates.
The evidentiary constraint binds earlier than either: a board cannot consider a comparable sale occurring more than 90 days after the valuation date, closing the usable window for a January 1 lien date around the end of March.
Our Contra Costa county guide links the county's own assessment appeals page.
The disclosure
A California Assessment Appeals Board equalizes the roll by reducing or increasing an assessment under Revenue and Taxation Code section 1610.8. We screen for that exposure before filing, do not file where we see it, and withdraw if it emerges.
To have your Contra Costa assessment checked against the county's own records, start a free analysis. If the value is already in line, we will say so.