Błędnie: Linda is a good flatmate. She lives next door and she always looks after my flat whenever I leave for a couple of weeks. 

Poprawnie: Linda is a good neighbour. She lives next door and she always looks after my flat whenever I leave for a couple of weeks. .

"Flatmate" to 'współlokator', osoba, z którą dzieli się mieszkanie. "Neighbour" to 'sąsiad'.