What kind of base or pad does a swim spa need?

Weight is the deciding factor. Empty, a swim spa may weigh 2,000-4,000 lbs; filled with water and occupants it commonly reaches 15,000-30,000 lbs, spread over the whole footprint.

What that means for the base:

• Concrete pad: The standard recommendation is a 4-6 in reinforced slab on a compacted gravel base, poured level and extending under the entire cabinet. Most manufacturers require this to keep the warranty valid.
• Existing patio: Usually not sufficient. Typical residential patios are thin, unreinforced, and poured for foot traffic.
• Decks: Possible only with engineered support. Have a structural engineer specify the framing before delivery.
• In-ground or partially recessed installs: Require a retaining structure and access space for equipment service — plan a service access panel on at least one side.

Level matters as much as strength. An out-of-level cabinet stresses the shell and can cause skimmer and cover fit problems.