On a pre-1900 build I often see them filled with mortar, at least here in Milwaukee.

Areas close to BlueOval City will likely see an increase in housing development, including single-family homes, apartments, and short-term rental properties for contractors and temporary workers.

@Jacques CaspiTo succeed in real estate investing, I’ve found it’s crucial to budget for unexpected expenses, focus on thorough tenant screening, and build strong relationships with contractors.

do you have a lender or are you building out of cash..

You're a contractor in the Bay Area, I'm guessing you're slammed with work.

The setback requirements to build in my county require 80' minimum wide street frontage per this neighborhood zoning.

You will want to insure a number of things as an MTR landlord: your interest in the building itself (owner's policy), your contents (owner's policy), your interests if a renter or guest is hurt on-site (landlord policy), and possibly flood, wind, lost-income policies.

And have you thought about building back utilities?

She is in a good position with the contractor who appears to be in direct contact with the tenants in terms of minor fixes as they arise. 

Also consider contacting houses that look vacant, simply door knocking in a neighborhood that you are interested in and talking to the homeowners in a genuine way, For Sale By Owners is another potential, using an App like Nextdoor or Facebook pages/groups related to local real estate and just telling the group what you are looking for, contractors also know about houses coming on the market soon and so do property management companies, and finally garage sales are a good way to find off-market properties as people prepare to sell.To Your Success!