Why does my book says: 강의실은 옆 건물에 있습니다 , instead of : 강의실은 건물 옆에 있습니다 ?

Hi everyone!!, well, I've got a korean book, and its name is : EASY KOREAN for foreigners 1, and in unit 4, i found something i don't understand, this is: In this unit, Book taught me that I should put the sentence in this order: Subject + Place Adverb + 에 + .... But there is a dialogue, which applies this rule, but at the end of this dialogue, it says :강의실은 옆 건물에 있습니다 instead of 강의실은 건물 옆에 있습니다 , and my question is : WHY DOES 옆 CHANGES OF POSITION? Please help me understand, please!!... Thanks a lot and I will wait for your answers...