I just got the form from the county for this for some improvements to my house, so I can answer. Permits for "improvements" are sent on to the county tax assessor, and they send you a form basically asking for the value of the improvement. Note that "maintenance" doesn't count, only "improvements" (and then only specific improvements; e.g., adding solar doesn't count). The value gets added to your Prop 13 value, but does not trigger a complete reassessment. So it's unlikely that any improvement would result in a 10x tax bill increase, unless you've owned the property for a long time (meaning that your Prop. 13 value was significantly less than the real value of the property).