Question: "What special considerations should I make for a hot tub or spa?"
Answer: Water weighs 62 lbs per cubic foot, so if you plan on setting your spa on top of your deck, when it's full of water and people, it's the equivalent of an elephant on your deck. It needs proper support. You will definitely want to hire the services of an architect, engineer, or deck design firm to help you with the additional support requirements for a spa. Your local Plan Review should also be able to help. (Your taxes pay their salaries remember. They can help you by working out some calculations to properly support your spa.) On the other hand, you may want to rest your hot tub on a concrete pad and build the deck around it. Here's where things also get tricky because you must insure that the deck is properly supported. And don't make the mistake of sinking the spa flush with the top of the deck. That first step into the spa may be so much lower than the deck surface that folks fall into the spa! (Which may be funny until it's your wife (or husband.))