Police believe the fire was deliberately lit.
Detective inspector Virginia Le Bas said the body was found inside the house and no one else was found on or near the property.
Multiple calls were made to the Fire Service at about 1.30 pm about a fire on Barrington Street in Somerfield.
The teenage girl’s identity will not be released until next of kin are notified.
A post mortem examination will be carried out tomorrow, and may provide clues as to whether the fire was cause of death or merely a cover-up.