WATERTOWN, Mass. (WHDH) -- An off-duty firefighter rescued an 18-year-old woman from a burning home in Watertown.

Deputy Chief Rob Iannetta and his wife were gardening in their backyard when they said they started to smell burning mulch. Not thinking much of it, they went inside for lunch and when they came back out, the woman spotted flames coming from the house next door.

“I ran down in between the pathway and I got into the driveway and I said, ‘Oh good, there’s no cars here, there shouldn’t be anybody home,’ but I went up and I rang the doorbell and expected the doorknob to be locked and it opened and I went, ‘Oh, no,’ I was hoping they forgot to lock the door. I just went in and yelled, ‘Hey, anybody home?’ and the daughter came down and said, ‘What’s going on?’ and you know, ‘You got to get out of the house, the house is on fire,’” said Deputy Chief Rob Iannetta, Watertown Fire Department.

The 18-year-old old was the only one home at the time -- her parents had gone out.

The damage to the house is extensive, but no one was hurt.

Smoking materials were allegedly found in the house. The cause of the fire remains under investigation.