The fire has been extinguished.
Initial investigations indicate the fire started in an unattended bedroom and quickly spread to the roof space.
The occupants were alerted to the fire by a smoke alarm sounding.
The building has sustained serious damage to the roof and bedroom.
The initial damage bill has been estimated at $500,000.
"The occupants were very lucky to escape the fire safely and raise the alarm," ACT Fire & Rescue commander Leigh Curtis said.
"This is another example of how working smoke alarms save lives," he said.