Emergency services were called to a house on Seaside Lane, Easington Colliery after a woman's body was found.
Durham Police and the North East Ambulance Service were called to the house on Tuesday night to reports of an unresponsive woman inside.
The 33-year-old woman was pronounced dead at the scene.
A Durham Police spokesman said: "Police and ambulance were called to an house on Seaside Lane, Easington Colliery just before midnight on Tuesday, September 13 following reports of an unresponsive woman inside.
"Soon after their arrival a 33-year-old woman was pronounced dead, her death is not being treated as suspicious and a report is being prepared for the coroner."