In the first example, you are saying that you hope that what is built on the site is as eye-catching as another building. The other building part is implied. For example, I might say, "I hope I am as excited as you." In this case, you are excited, and I am hoping to be at the same level of excitement as you are. It is understood that there is an 'are' at the end of the sentence.
In the second sentence, you are simply hoping that the building is 'eye-catching' or attractive. You are not comparing it to something else.
I hope that makes sense and good luck learning English!
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