For intrepid do-it-yourselfers, building a hot tub for your own personal use is fun. But, where many builders go wrong is with the foundations that the hot tub will eventually rest on. That is really the entire key to a tub that will last years and years.
The first thing to remember is that, while an empty hot tub weighs 750-1,000lbs, when full it can top 4,000lbs (2 tons). And that's before any people are in it. With six average-size adults, your tub might touch 5,000lbs. So, you've got to make your plans with that kind of weight in mind.
Another important consideration is access to the electrical source required to power most hot tubs (16-32 amps). This is true whether you are building a hot tub yourself or having a pre-made one installed. Just try to avoid getting so caught up in placing the tub in a spot you really love that you start to build a hot tub foundation and suddenly realize: "Uh oh! I forgot the power source!"
Once you've got these preliminary aspects in hand, your next decision will be how to support the hot tub. If you want to place it on a raised deck inside your yard, you'll want to figure out the size you need (add about two or three feet all around the tub to accommodate access). If the tub is six feet in diameter, for example, a 9' diameter deck is probably adequate.
Support the deck by digging down to the hard-packed soil (at least one foot deep), fill the holes with sand and top with gravel, then be sure to level each hole. You'll need pier posts (sometimes called 'footings') to go on top of the sand/gravel underpinning. Ask your local building supplies pro what you need.
Keeping all the footings level is extremely important. You'll need at least 6 posts (more if you're installing a large hot tub). It's better to err on the side of caution by digging and embedding more posts than you think will be needed as a bare minimum.
Now you're ready to build (or buy) the decking and the hot tub. If you're careful and factor in the proper weight distribution, you will be able build a hot tub foundation in a weekend that will last for years.