How to decide on the size of an ADU?

[Jeff Cliff]

Hello,

I am currently rehabbing a property in central coast California. In addition yo rehab work I also want to build an ADU. The lot size is approximately 10K sq.ft and the existing house size is about 1670 sq. ft. So the lot size is not much of a constraint especially with new offset requirements for 2020.

Now my question is strictly from the financial point of view, how do I go about deciding the size (sq.ft.) of the ADU? What are some of the considerations I should make?

As a side note, for now this house will be an investment property and will be rented out for a long time and possibly I will keep a room on the upper floor for myself. Eventually, I would retire in this house after 20-25 years.

Thanks in advance for your response.