If I demo my detached accessory structure that is less than 3 feet from the property line, can I rebuild it as an ADU? What if I just want to add on to it?

No additional setback is required for an ADU if constructed in the same footprint and with the same dimensions (including height) as the existing structure. An expansion of no more than 150 square feet beyond the existing dimensions can be allowed for ingress and egress purposes only.