An indoor hot tub should be chosen based on essentially the same criteria you would use when choosing an outdoor hot tub. More important than the tub itself is the room in which the indoor hot tub will be installed, since drainage, electrical outlets, steam build-up, and adequate support for the weight of the tub will all become issues during installation. The indoor hot tub should be chosen based on the number of people who will regularly use the tub, what types of features you want included with your tub, and what kind of aesthetic you want your hot tub to fit.
Much like the outdoor versions of hot tubs, an indoor hot tub must be placed on a solid, even surface. The weight of the tub will be significant enough to cause damage to floors that are not meant to support that much weight, so a concrete floor is best. A drain in the floor will make your life easier as well, since water will splash out of the tub during and after use. A drain will help prevent damage to the floor and help maintain a sanitary room in which the tub will reside.
For a more permanent indoor hot tub, a wood structure can be built, and an acrylic tub can be inserted into that structure. This is a more labor-intensive project, and the finished product will be a permanent fixture of the house. It is important to properly waterproof the structure to prevent mold and mildew from accumulating, and to prevent rot in the boards used to construct the tub structure. Concrete can also be used to make the tub structure, and while this is a more structurally sound choice, it is also more difficult to build. The finished product will also be much heavier, so the floor will have to be strong enough to support the weight. The plumbing and wiring that will be run to the tub may also become more difficult to access.
Many people choose to build a room around the indoor hot tub, both to make installation easier and to ensure the tub cannot damage the home should a leak occur. Hot tub houses can be built to any size or shape, depending on the size of the yard or space in which the structure is being built, and it allows the builder to customize the structure to accommodate a variety of needs and desires. This is a great option for someone who does not want the hot tub to be inside the house, but who still wants the convenience of an indoor tub.