former residence; old home
Refers to a place where one used to live; often used in formal or written contexts. Not typically used for a house one just moved out of yesterday; implies some passage of time.
He visited his former residence and reminisced about old times.
I heard that Soseki's former home was around here.
旧居 refers to a former residence one has left, the opposite of 新居, the new home one has just moved into.
Compound of 旧 (former, old) and 居 (residence, dwelling). The reading is on'yomi for both characters.