Checking with the departments one by one for each land listing that looks promising seems very inefficient.Regarding costs, i hear build costs of containers vs stick built being similar, but I had wondered if that is taking all aspects into consideration like build time, foundation and site prep requirements, etc. yes, steel cutters and welders are a special trade but the structure can arrive pre-built and potentially just sit on concrete pier foundations if the soil and zoning allows.

Is the answer just to work fast or are there tips of the trade and best practices? 

A real estate agent / brokerage trade or business doesn't constitute a 'specified service trade or business' under IRC Sec 199A.Best to talk to an attorney for the legal perspective, a tax CPA/EA for the tax perspective.You have Disregarded Entity, C Corp, or S Corp at your disposal for tax, assuming you're the 100% owner.

No idea about your state but the two things that tend to trip up basement ADUs when they are trying to go “legit” are adequate emergency exits ala windows and ceiling heights. 2nd means of egress?

Have some emergency funds. 

BUT, my CPA is telling me that in order to claim partial disposition deductions on my newly acquired office building, I needed to place it "in-service" PRIOR to disposing of any assets because per the Partial Disposition rules (listed within the final Tangible Property Repair Reg's released in 2014), assets must be used in a trade or business or rental activity to be claimed. 

An individual qualifies as a real estate professional for the tax year if a. more than 50% of the personal services performed by the taxpayer in all trades or businesses during the tax year are performed in real property trades or businesses in which the taxpayer materially participates; , The rental or any other activity that you do not materially participate cannot be counted. andb. the taxpayer performs more than 750 hours of service during the tax year in real property trades or businesses in which the taxpayer materially participates., the rental or any other activity that you do not materially participate cannot be counted.

Then start working on establishing credit and saving up for a down-payment and an emergency fund.

Not a mom & pop but a big time company with professional ads, marketing, administrative staff etc....Let them handle all the nonsense and churn that is natural in the construction trades. 

My background is Accounting, Finance, Equities trading/Financial planning and worked in Commercial leasing in Manhattan back in the early 90's.