Spa construction expenses can vary greatly according to the type of building you are considering. If the infrastructure for plumbing electrical and HVAC are in place or are included as part of the landlord's build-out expense it can save thousands upon thousands of dollars!
An average for new construction costs in a building shell where the infrastructure is stubbed out just to the spa site (not specifically to each room, area, etc) is a cost of $100 to $175 per square foot. If it is a new construction which requires construction on a piece of raw land, the average construction cost is $200 per square foot and on up.
If you are leasing the spa location in a new building under construction, many times the builder will "build to suit" and recoup his expense in higher leasing fees. In this case the owner will bring the plumbing, electrical, and HVAC specifically to each site (i.e. massage rooms, etc.) as specified by the spa owner. The spa owner, however, is still usually responsible for all fixtures such as sinks, cabinetry, lighting, etc.