Ask the manufacturer.

If this going to be a poured in place concrete vessel, you will need to consult a structural engineer.

Also, it must conform to the current pool code requirements, i.e. VGB compliant suctions inlets, and also National Electrical Code requirements, i.e. section 680

Mine is interior, but the manufacturer provided us with all the design documentation for ours, which included engineering report.

I apologize-I read the original post a bit too quick.

I understand your question.

Consult manufacturer or a structural engineer. A properly designed support system is required to handle the swim spa with or without water.

Any concrete support system will still have to conform to the NEC 680, (particularly bonding) as the structure will most likely be reinforced with rebar, because of it's proximity to the swim spa.

Thank y’all. Unfortunately the manufacturer is not much help (master spa). Only recommends a 4” concrete slab. No problems here, but since we are doing it partially in ground trying to figure out the best ways to do the “walls” with cinder block. Reviewing basement diagrams, there is a footing that is installed first and then concrete poured in. For leveling purposes, this actually seems more difficult. I have also seen diagrams where the concrete slab is poured first. It will be around 30” deep on one side and 50” deep on the other (its a slope). Nothing load bearing on top of these walls, but they will act as retaining walls. I’ve also looked at various drain options. Thoughts?

We have a licensed professional electrician coming out this week to review from their perspective. So will absolutely do to code (I wouldn’t ever diy electrical).

A year ago, I also really wanted a small Master Swim Spa, only to find out Master Spa only sells to deliver. The only local spa installation contractor insists it installs Cal Spas only, no Master Spa. Cal Spas, located in Pomona, CA, is having horrible reviews with its severe manufacturing issues & problems, & anyway deliveries are delayed for a year at least. I agree such a big 15 ft tub sticking up 5 feet above the cement pad is an eyesore, and I would want a partially sunken installation. I am giving up and now wondering if building one swimming lane, would be easier to avoid the sunken vault issues, which the local contractor with Cal Spa says adds at least $10,000 (for a swimspa that does not work, this makes no sense)