A pre-fab spa pad is an excellent option for hot tubs and spas. It is less expensive, faster, portable, and does not require a permit to put in. The best of these is the EZ Pad which has sold over 100,000 units since 1999, with zero reports of problems. With the exception of spas that are assembled on site, any hot tub or spa can be placed on an EZ Pad, including larger swim spas. I have personally read many warranties from spa manufacturers with the express purpose of finding any reference to requirements for concrete, and have found none. These products exist only because they do not violate spa warranties. If you want a fast, inexpensive way to install a hot tub, an EZ Pad is a great choice.