Casitas and basements are two different things. A proper casita is a small, self-contained wing of a home or separate building containing a restroom, sleeping/living quarters and possibly a kitchenette. A basement can be almost anything. You will find casitas to be common in newer construction and in developments where there are larger homes. Casitas (proper ones, anyway) are typically found in your more expensive or custom home.
Basements in Arizona add quite a bit to the cost of a home. Again, you are looking at more expensive properties if you want a basement. You won't find casitas or basements in Glendale because those are primarily older homes. You will want to focus your search in areas that have recent construction: Scottsdale, Mesa, Queen Creek, Chandler, Gilbert, Cave Creek, Desert Hills, etc.