Sauna pool covers three quite different setups: a small plunge pool built for cold immersion, a cold plunge tub standing beside a cabin, and an actual swimming pool a sauna happens to sit next to. They cost wildly different amounts and only two of them do the job people have in mind.
The cold plunge, which is what most people mean
A small, deep, insulated vessel held near or below fifty degrees, sited a few steps from the cabin door. It is the cheapest of the three by a wide margin, it needs no excavation, and it does the thing the pairing is for: a sharp cold contrast immediately after heat, at a temperature a swimming pool never reaches.
The built plunge pool
A small in-ground or partly sunk pool built for cold use, with a chiller, a cover and proper plumbing. It looks permanent and beautiful beside a cabin and it costs what building a small pool costs, which is an order of magnitude more than a tub. Choose it when you are already doing landscaping, not to solve the cold problem.
The swimming pool next door
A swimming pool held at swimming temperature is not a plunge, because the contrast is what does the work and a pool is nowhere near cold enough for most of the year. It is a pleasant place to cool off after a sauna and it should not be bought as one. In winter, in a cold state, that changes entirely.
Siting the pair
Close enough to reach in a towel, on a surface that will not be slippery when wet and cold, with something to hold on the way in and out. Drainage matters more than people expect, because the ground between a cabin and a plunge gets wet every session. And put the plunge where you will see it, because the one you walk past is the one you use.
Questions people ask about sauna pool
Do I need a pool beside my sauna?
No. A cold plunge tub does the contrast job for a fraction of the cost and reaches temperatures a swimming pool never will.
Is a swimming pool cold enough?
Rarely, outside winter in a cold state. The contrast is the point, and swimming temperature is too warm for it.
What matters most in siting them together?
A short walk on a non slip surface, something to hold, and drainage. The ground between the two is wet after every session.