Considering building a casita (small home) as an investment property

My comment is about where to invest/build and then knowing your immediate competing homes.

I've designed and built 2 of my homes in Puerto Rico and bought 2 new construction homes.   If your goal is for it to be an investment property, first you need to know who your likely customer will be that rents your home.  A 1 bedroom or 2 bedroom home will likely have the most competition. My homes are all 3-5 bedroom and only short term rentals so no stays longer than 28 days. My homes also have diverse guests so that sometimes it's people from the States, Canada or Europe and other times people who live in Puerto Rico who want to rent a house for their vacation and not leave the island.   The best suggestion I can give you is to use airbnb.com or homeaway.com and just search areas that you may consider to build and see how much they charge per month (each month the price might change) and how booked they are each month and then decide if you will clean it or hire someone to clean it, depending on distance from your own home.  You would also want to account for the cost of replacing items and things like electricity, internet and water expenses (linens, towels, toilet paper, cleaning supplies, etc also add up quickly) and keep in mind, you need to furnish the home... what makes your vacation home unique and different is also important. Guests book online so the photos and appearance of the front of the home as well as the landscaping and colors you use are important.   

As far as builders, I would look at their work and talk to past customers about their experience. Also, make sure your contract requires them to finish by a reasonable date or there should be a penalty that reduces the total cost to build it.  Basically, look for contractors who work hard and have happy customers who can tell you that the work was completed on time and as expected.

Make a few files, do some research on nightly rent, cleaning fees, and overhead monthly expenses.  Start with narrowing down your location and know your competition.  Maybe even stay at a few vacation homes to see how they operate.   Join-A-Join.com is local to Puerto Rico and a vacation booking company, you can talk to them as well and get an idea of where there may be more need and what the rental season is in each location.  Some months you may lose money so you will need to make it up with the good months.