Look at the two sentences. In the first, you have the word ‘a’ (which is singular) in front of 4800 square foot (foot is also singular); and all these words modify the word ‘home’ (which is also singular). In the second sentence, there is no ‘a’ and you are talking about how many feet there are in the living room. Consequently, the plural form is used. It’s the same with “He is six feet tall” vs. “He is a six foot tall basketball player.”