I only use foam in high-end houses and when it's required to meet energy codes (for example, a house I just built in Georgia, the roof was framed with 2x4s instead of 2x6s, and the only way to get the required R-30 was to use spray foam).

Even Though (crawl space vents) are still a code requirement in most areas, I don't believe they are the way to go. 

Did you find that there were any unique obstacles to building modular homes other than the obvious building code, zoning etc. that you encountered in SoCal that you wouldn't elsewhere?

So many glitches in bad code these days.

Call code enforcement. 

@Keith Nelson the answer to your question lies with answering two more questions: (1) how much do you know about taxation in relation to rental real estate; and (2) how much time are you willing to put into researching the tax code to ensure your return is compliant with the regs?

Net listings are discouraged by the National Code of Ethics (Article 1).  

He started putting up drywall before my electrician was even close to being done as whole house needed to be brought up to code.

And I did some market research on the area and the zip code itself is operating at about a 15% occupancy.

Sounds good on paper but always noise issues, shared utilities, yards, garages everything becomes a bone of contention.For conversions, there are codes issues with the separation of living quarters and fire suppression.