Tax implications of building an ADU in Berkeley

I'm curious what the other replies are. Last time we pulled a permit for a kitchen remodel, I got a questionare from the Assessor about the improvements. I would think building an ADU would least require the value of the ADU is added to your assessment. 

https://www.acgov.org/assessor/about_property_assessment/supplemental-a… (<- note that New Construction triggers something, but did the ADU ordinance exclude supplemental assessments?)

This is what I was suggesting - phrased better from a Santa Cruz guide to ADUs

" The base value of the land and existing improvements will remain unchanged, and an increment
representing the change in property value because of the new ADU will be added via
a supplemental assessment. (See your property tax bill for information on the current
assessed value of your property.)"

I built a 1BR/1BA ADU in Oakland (Alameda County is responsible for assessments, so your experience in Berkeley should be similar) and had my property assessment increase by about $125k.  The ADU replaced the detached garage that had been on the site (same square footage) and was done with permits, which triggered the county sending me a form to describe the project.  I spoke with the assessor's office after receiving the increase, and the person handling my file stated that they use a formula to determine the increased value of the property and that my increase was consistent with adding an additional bedroom and bathroom.  Considering the cost of building the ADU and my perception of the ADU's effect on the value of my property, it seemed like a fair assessment and I didn't challenge it.  Prop. 13 limits the county's ability to reassess, so they are not allowed to undertake a full re-appraisal of the home just because you build an ADU.

For home renovations generally, the reassessment is just to add the value of what you've built onto your existing assessment. So if you build an ADU, your tax assessment will be existing home assessment (still subject to Prop 13) + value of new ADU. Rumors seem to fly that the entire property will get reassessed if you build an addition or renovate with permits, but that's not the case. You will be taxed on the new unit, of course, but the assessment of your existing home will remain unchanged.

Contrary to what you’ve seen, I’m pretty sure the answer is “yes”. Just as additions / certain renovations to your property trigger a reassessment, pulling a permit for building an ADU would similarly result in a reassessment. We’re building an ADU and are assuming property taxes will rise; we’ve also been told “probably not by much”, though who knows there. 

Property in California is subject to reassessment whenever there is a change of ownership or new construction. An addition, adjacent dwelling unit, or remodel is considered new construction. The reassessment will add the additional new market value of the improvement to your existing base year value. Mere repairs do not constitute new construction. Also certain items, such as foundation retrofit for earthquake protection, and perhaps solar, are exempt from reassessment for purposes of public policy.  Check the county assessor for your county.