Błędnie: The house needs renovating and redecorating. It's roof needs mending in the first place- water's been leaking for ages now. Poprawnie: The house needs renovating and redecorating. Its roof needs mending in the first place- water's been leaking for ages now.
"It's" to skrócona forma "it is". Formę dzierżawczą zaimka "it" pisze się bez apostrofu- "its".