Posting for traffic - please help me find this book

[Pebblebeach15]

Please could you help . I really want to reread this book , but have forgotten the author and title.
It begins in the Summer with a group of friends on a day trip to a lake . I think they have just graduated . They discover a deserted cottage and decide to live there . They end up staying a year , becoming self sufficient through the hard winter . The main character’s sister visits and drowns . In the end it turns out that one of the male character’s mum lived a just a short distance away over a hill .