The Water's Lovely by Ruth Rendell
'Weeks went by when Ismay never thought of it at all. Then something would bring it back or it would return in a dream. The dream began in the same way. She and her mother would be climbing the stairs, following Heather's lead through the bedroom to what was on the other side, not a bathroom in the dream but a chamber floored and walled in marble. In the middle of it was a glassy lake. The white thing in the water floated towards her, its face submerged, and her mother said, absurdly, "Don't look!"'
The dead man was Ismay's stepfather, Guy. Now, nine years on, she and her sister, Heather, still live in the same house in Clapham. But it has been divided into two self-contained flats. Their mother lives upstairs with her sister, Pamela. And the bathroom, where Guy drowned, has been demolished. But Death will rear its ugly head once more...
Ismay and Heather get on well. They always have. They never discussed the changes to the house, still less what had happened that August day. But as their love lives start to develop, someone is murdered along the way, and long buried suspicions re-emerge with potentially tragic results.