I recently completed one on a multifamily property and found it to be a cost-effective approach.That said, there are a couple of additional factors to consider beyond the code differences between a garage conversion and new construction:Coastal Overlay Zones – If your property is in a coastal overlay, building a ground-up ADU may require Coastal Commission review, which can add time and complexity to the process.

My budget is in the $200-250K range.Please feel free to let me know if additional details would help, and thank you in advance for your time and advice! 

I've owned the house for few years and have spent a lot of time trying to design an addition to add value to the property, but the current floor plan is really not conducive to realizing a return on that investment (tiny living room, low ceiling basement, poorly placed basement stairs, etc.). 

Beyond that, networking with other investors in the mobile home park space could open up some doors for partnerships or additional funding ideas.

I currently have a homeowners plan with Allstate and plan on asking them if they have something like a short term rental rider plan that I can add in addition to my current plan.

In addition to the different tracks @Chris Seveney descrubed, there is also good information on creative financing and seller financing (including what to avoid).

Additional Considerations: Costs & Market Changes•Ongoing Maintenance & Housekeeping: STRs require regular deep cleaning, restocking of supplies, and maintenance.

In addition to STR'ing the main house, he is building 2 more units (ADUs) on the back side of the property.

In addition I am giving him freedom to explore (to a certain extent) on my dime.

Get the property in your name, make sure that the seller is an additional insured.