Depending on your market it may not make financial sense at all.

Expect a custom ADU to take 9–18 months, with 3–6 months for design and permitting and 6–12 months for construction, depending on complexity and contractor availability, and the speed of processing permits.

 @Sri Reddy it all depends on your goals.

This would depend on whether your property is overvalued by the tax assessor and in turn you are paying additional taxes on that additional valuation.

I think it will also depends on state laws and a lot of information not present but the HOA rules and insurances are two different things.

While this is municipality dependent, if the contractor is not licensed you likely cannot obtain the necessary permits and approvals.

That will be an important piece to this depending on how you plan to source your capital.

Whether the lender decides to do anything, and exactly what they decide to do depends on the lenders particular strategy, target ROI, risk tolerance, current interest rates, interest rate of subject mortgage loan, etc. 

It's not what I would call a super rough, probably different street by street, and would most likely depend on the neighbors right around the property. 

Many of also argued that forcing out many STR's will impact our tourism revenues, which we're very dependent on, as many travelers won't come here if they have to stay in a hotel.