Please take this as constructive criticism, as this is good for such a young writer.

Why does he ask "did someone live here twenty years ago?" He would more likely say "who lived here before us?" He must know they didn't build the house themselves, and if not then obviously someone lived there before them. The question is who were they? Mum could say in reply exactly when the last occupants left.

Work one your grammar and be sure you had some suspense . But it is pretty good.