Their new building is very unattractive.

unattractive 是由前缀 un-（表示"不、非"）和形容词 attractive（有吸引力的）组成的复合词。attractive 来自动词 attract（吸引）加后缀 -ive 构成形容词。unattractive 字面意思是"不吸引人的"，即"难看的"。