Fire crews were called to a blaze at a bungalow in Northorpe, near Bourne on Thursday night at about 7.45pm.
The blaze happened at a South Kesteven District Council house in Almond Court, Northorpe, on Thursday at about 7.45pm.
The tenant, a 63-year-old woman, was unhurt in the fire but much of the bathroom was damaged.
A district council spokesman said: “There was extensive damage to the bathroom of the bungalow and the fire service’s view was that it was caused by a discarded cigarette and this was confirmed by the support unit on site.
“The tenant, who was unhurt, called our 24-hour control centre using her intercom unit but the neighbours were unaffected.”