Ours was built with the purpose of a hot tub, the joists spacing & size were such that it would take one, it's also got a underground cable going up there on it's own breaker, but we've always been put off by the expense of running one.
We have a 'blow up' hot tub - It's from Intex and the material is really thick. Yes, the heating cost is about £30-£40 a month, and then chemical treatment, but it's been good. With these cheaper ones you are best running them constantly - initially we put it away over winter, but the general damp in the garage knackered the motor. That was replaced under warranty, so I built the gazebo, and it runs 24/7. Costs less if you are just 'maintaining' a temperature, rather then letting it go cold, then re-heating over a day. Also enables the chlorine to work properly.
They aren't being flagged up ! 
