A large fire caused heavy damage to a vacant luxury home under construction in south Oakville Wednesday.

The flames apparently started in the garage of the mansion on Ennisclare Drive West, south of Lakeshore Road East near Ford Drive, just after 5:45 a.m.

Authorities said work on the house was nearly complete when the fire broke out -- the exterior had been finished and the interior was being painted.

"Fire was showing on the second floor when crews arrived. They attempted entry into the home to try to extinguish the fire. They were forced back out and the fire extended into the attic. It then burned through the roof and the attic," Deputy Chief Andy Glynn told CityNews at the scene. 

The home was so severely damaged, it was unsafe for firefighters to enter. They fought the blaze from the outside. 

The cause of the fire is not yet known. The Ontario Fire Marshal's office is investigating.

Damage is estimated at $2.5 million.