I agree with the above but have to admit that sometimes (many times) it is a hard call. What's a patio... a 10 x 10 concrete slab somewhere in the back yard. What if it's broken up? What if it's elaborate? What if it's just some stones collected from the yard and placed attractively and they have some yard furniture and a BBQ.
MLS doesn't usually quantify the patio, decking, covered porch. Sometimes they use enclosed porches in the GLA. Patios and decking are at the rear of the house and most often cannot be viewed from the street. It's fine if there are rear yard pictures in MLS. Most of the time there are not. Because of this, my adjustments are not very large and they are somewhat generic. They also increase or decrease based on the quality and price range of the market. Large, upscale homes with many amentities "should" have some sort of large, upscale patio or decking system so my adjustments hit them or credit them much harder for any lack. Small, low end houses offering basic shelter, get much lower or no adjustments. The range in between gets my more generic, "standard" adjustments.
I also admit that I do not carefully measure decks and patios (I do measure enclosed porches, sunrooms, atriums, etc.) I generally estimate the size and shape and location relative to the residence and sketch it in based on that. I'll also note the condition and quality and make field notes like small, medium, large or very large. There's just not enough good data in my market to do otherwise. Keep in mind that I work in a large, sparsely populated, rural/semi-rural area. The low end of SFR's is about $75k and the sky is the limit at the upper range (as much as $2-3 million, with a few at $10 million +.)