The incident happened at 3am on Saturday the 6th of May at the property in Bedford Hill.
Details of the incident have only just been released by officers.
The woman in her 20s had fallen asleep in her living room when she was woken by an unknown man and sexually assaulted.
She screamed and he fled the scene.
The suspect is described as black, aged in his late 20s to early 30s, slim, but not athletic build, about 5ft 7ins with short afro hair.
At this time the inquiry is being treated as an isolated incident.
