Crews were called to the corner of Vernon and Bowdoin Street around 4:15 a.m.
Firefighters found heavy fire coming from underneath the building and inside the home when they arrived.
Two people and a family pet were inside, but were able to make it out safely.
The Bangor assistant fire chief says the owner smelled smoke around 7 p.m. Monday, but an initial investigation didn't find anything.
He believes the fire started on the outside of the building.
"There was significant damage to the mud room, it only made it into the main home through the kitchen area and it's a minimum amount of damage," said Bangor Assitant Fire Chief, Dennis Nadeau.
The cause of the fire is under investigation.