WE enclosed our lanai area (260sf) and added an additional 150sf hobby room off the master bedroom and connected it to the now enclosed lanai with a pocket door. WE also took out the sliding glass doors to open the now enclosed lanai to the main family room. WE also wanted to add heat/ac to both new areas which addeed approx 400sf....and we wanted the heat/ac to be directly ducted to the main furnace...not simply tapped to the nearest duct in the family room.
When our contractor took the plans to the county for approval, they required him to present a completed heat/ac survey. The contractor then had to contract with Munns and this survey is not just someones guess...the survey considers the type of windows you are installing (low e insulated, tempered heat gain etc) the direction of the new space in relation to sun direction and compare that to the capacity of your existing AC unit or heat pump if you have all electric.
It costs a couple of hundred bucks to have this survey completed but we certainly liked the results and it has proven more than satisfactory for us. Every house will be different and if the survey is not required to secure your building permit it certainly will provide you peace of mind. Believe me saving a couple of hundred bucks to install something that will not adequately cool or heat the extra space will only make you more uncomfortable (and miserable) once you start using it. AS you probably already know an enclosed lanai facing any part west really gets hot in the summer
BTW: If the existing Ac is not big enough to handle the new addition, you donot need to replace the exisitng AC. There are new units out today that attach to the wall and vented outside with a separate outside compressor and are fairly reasonable and work pretty good. One that comes to mind is the Mitisbuti "Mr Big".
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