One very big consideration is going to be the size of your spa. If you've got a 450 galloner or more, that'sa bit of weight.
The first thing to do is pick a spa. Once you have picked a spa, you need to find out from the dealer what kind of bases will and will not void the warranty. For example, my HS grandee could not be shimmed and could only be installed on a slab or a reenforced deck because of how heavy the thing is.
My friend has a sundance cameo and he had 2' - 3' (2' in front, 3' in back to make it level) slab poured to hold the tub. It works just fine for him, and he is on year 3 with it. But, he had a professional pour it.
I did my slab myself, but it was only 3-4 inches off the ground. To dig it I:
1. Checked with building code to make sureyou don't need a permit. Since mine was only 8'x9' I did not needone.
2. Dig a 8" base with a 12" moat (footer) all around it.
3. Tamp it.
4. Drop in 4" of drainage rock.
5. Tamp it.
6. Cover the rock with thick plastic sheeting.
7. Weave/tie a .5" rebar mesh with rows and columns every foot. Use holders to hold it 2" off the rock.
8. Fill it in with concrete.
That *might* be the same process you use to build a 1' or higher slab, but you might need deeper footers, or more rock, or you might be advised to pour the concrete in 2 steps.
If you call a contractor out to price the work, they will usually tell you what they need to do, usually from the standpoint of justifyingthe expense. Then, if it is too expensive, you have alistof what you would have todo to build ityourself.
The manual labor, though, was impressive.
Good luck.
-Ed
