Lanai Enclosure
 

I found out that you can enclose your lanai in either glass or acrylic sliders and as long as you don't run vents for AC/heat, it's not considered habitable therefore it's not assessed as an addtion.

So this being the case, has anyone used portable AC units in their enclosed lanai's with success? Same with portable heaters - do they work?

If you live in the Marion County portion of the villages, code requires you to "heat" the addition thus it is considered habitable and is taxed as an addition. We put on a glass enclosed sunroom (and love it) and were required to add heat. In our case the existing furnace was adequate so that all the contractor had to do was to tap into the existing ductwork.