Not difficult, just different and critical.  

Contractor: Finding contractors that don't cut corners and overcharge is EXTREMELY difficult.

When the sq ft numbers for pre-fab are quoted, they typically don’t include the costs associated with site work, foundations, and utilities; they are only quoting the costs of the ‘boxes’ that get dropped on site.Many jurisdictions still haven’t accepted pre-fab buildings as an approved construction type as well, which can lead to a longer and more difficult permitting process.I would do my due diligence for sure before going down the prefab route

It can be difficult to collect tenant portions at times.

@Forrest Brown,In addition to the other advice and variables you're juggling here, you should know that California makes it a little more difficult to sell and 1031 exchange out of state. 

You must be VERY careful when building cabin-style homes to STR, as they may be difficult to refinance.

Be wary, each fund has risks (some are "levered debt funds", some lend in second position or at higher LTV than you will like, some may lend in regions that make it difficult to foreclose, some may charge high fees or offer low returns). 2) To ask them if they have any notes for sale - Be warned, lenders may try to offload the loans that they don't want to keep on their balance sheets.

Take the time to sit and chat go through difficult scenarios to ensure that everything is hashed out.

You will want to have some money handy and work with a contractor and there are construction loans but it may be difficult if this is first time without a partner who is experienced and has a nice resume of builds.

As far as how to find a property that has a basement apartment or a property that you can convert a basement into an apartment… this may be somewhat difficult.