You may want to get another AC recommendation. Wey=ther your existing ac unit can handle the additional space is very important for comfort. Check the size of your unit it should read something like 30XXX, 36XXX or 42XXXX and then compare it to your neighbors house of the same size.
Then you have to consider in terms of what you are doing, ie what type of glas will you be using (low e insulated or plain), direction of total sun exposure with emphasis on western sun, and sq footage addded. The county made my contractor do what they called a heating and ac SURVEY, Mums did it ($150). Also make certain the new AC/heat ducts are connected direcly to the furnace not simply an extension of the closest duct outlet.
We added 400 SF and did not have to change out or add to our heat/AC system but we had 1/2 ton more capacity than our neigbors when built for we had a slight (new build) expansion.
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