The house was built in 1976, so if it was before 1989, It sounds like there might not be a way to verify.So my questions here are, do you get an architect and/or engineer to come look at it to make sure it is built to code and then apply for an as is permit from there or is there another course of action to take?

I didn't mention, that I'm a design architect by day job.

You should also consult a local architect because zoning regulations, setbacks, build-to, height requirements, parking requirements, etc will all vary.

Do you have a builder, engineer, architect, etc.5. 

Not sure where your at in the process, but before you even begin you could blow through a $100k on architect, asbestos testing/remediation, boring/soil test, drafting of plans, numerous surveys, expediting, etc. 

I worked with other syndicators, architects, engineers, contractors, lenders, insurance agents and many more team members.

the city won’t want to know that you plan to connect the new addition to the ADU, but ask a competent architect with experience in adding ADUs, and see if he thinks you can design it so later on the addition would flow with the ADU.

Your architect is going to need the survey, geotechnical report, and program (unit mix, office sf, design styling, desired leasable sf, etc) as they are getting started.

My thinking was that if these two entities shared the same LLC, but had distinct DBAs, then my expenses with the real estate (land purchase, engineering consultants, architect and construction costs, etc.) might be offset somewhat by my consulting income.  

There are folks versed in permitting and land use in your area - I'd start with an architect ... or Civil Engineer who advertises themselves as a consultant.