The ACT Emergency Services Agency said the occupants were lucky to escape the fire, which started in the Minor Wylie Crescent home shortly before 9pm on Monday.

"Initial investigations indicate the fire started in an unattended bedroom and quickly spread to the roof space," the agency said in a statement.

"The occupants were alerted to the fire by a smoke alarm sounding."

ACT Fire & Rescue commander Leigh Curtis said the building's roof and the bedroom were badly damaged. He estimated the damage bill at about $500,000.

"The occupants were very lucky to escape the fire safely and raise the alarm," he said.

"This is another example of how working smoke alarms save lives."