The fear of losing Prop 13 protection is the single most common reason California homeowners hesitate on ADU projects. The fear is understandable — Prop 13 is one of the most valuable financial assets a California homeowner owns. The good news is that an ADU does not trigger a full reassessment. The nuance is in how the new value is added.
Blended, not reassessed
Building an ADU does not trigger a full reassessment of the main home. Cyrus Poudat reassures California clients that only the new construction is added to the existing tax base. The decades-old Prop 13 assessment on the original home stays untouched.
How the new value is set
The county assessor adds the construction cost — not market value — to the existing assessed value. Cyrus Poudat sees California ADU tax increases of $1,500–$4,500 per year on a typical $300K–$450K build. That is the 1–1.25% blended tax rate applied to the construction cost basis.
JADUs are nearly tax-free
Junior ADUs carved out of existing square footage add minimal assessed value because no new conditioned space is created. Cyrus Poudat recommends JADUs to California seniors who want rental income without a tax shock.
Garage conversions
Converting a garage to an ADU adds modest assessed value — the improvement cost, not the full market value of the conditioned space. Cyrus Poudat sees California garage-conversion tax bumps of $800–$1,800 per year.
Selling separately changes everything
Under AB 1033, selling an ADU separately as a condo triggers a full reassessment on the newly created parcel. Cyrus Poudat models the after-tax math for California sellers before recommending the path — sometimes the higher tax basis is absolutely worth it, sometimes it kills the deal.
Insurance impact
Adding an ADU also raises homeowner's insurance — typically $400–$1,200 per year. Cyrus Poudat folds this into California pro formas because lenders increasingly want it modeled.
The honest math
On a typical $400K California ADU renting at $3,000/month, the $3,000 annual property tax bump is roughly 8% of gross rent — meaningful but easily absorbed. Cyrus Poudat builds it into every California feasibility model so there are no surprises when the first reassessment notice arrives.