You will be able to do this optional quest once you have completed the Novigrad Dreaming quest, as long as you let Sarah stay at the house. Some time later, you should be able to find the "Beware! Haunted House & Cheating Merchant!" post on the Hierarch Square Notice Board.
While strolling the bustling streets of Novigrad, Geralt came across a most intriguing notice. It suggested the man named de Jonkheer had sold some poor fool the house which Sarah, the lovable yet mischievous godling, used as her dwelling and playground. The notice made it clear the new owner was not amused by his unwanted tenant's pranks. Geralt decided to check up on Sarah and see how she was doing...
