Just because the costs might be in the furture does not warrant not budgeting for those known expensesThen there is the misc costs of asset protection, bookkeeping, unexpected utilities (such as between tenants (tenant turn over) or failure not attributable to the tenant such as slab leak).  

What type of MEP work are you putting into the design for getting the sewer, water, power, and internet (assuming you're not extending gas) to the inside of the building? 

The gas line to the stove looks awful and dangerous to someone who has kids.

Oil and Gas offerings are what I have heard a lot of people move too.What I always caution people who buy vehicles is: Are you buying it because you need a new vehicle or are you buying for a tax deduction.

Ok @Derek Stevens, now we are cookin' with gas!

CA has the highest gas tax in the country. 

Mark down all the multis you see by looking for two or more doors in front or multiple air handlers outside or gas meters.

Are there separate gas, electric and water meters for each apartment?

Are all 4 units metered for gas and electric separately or on master meter? 

Our electricity is already separate and we designed our ADU to be all electric so no gas.