Fire broke out Sunday evening in a Santa Rosa garage where a marijuana growing room was under construction, a Santa Rosa fire official said.

Firefighters were called to the westside Northfield Drive home at 6:05 p.m. The first engine arrived at 6:10 p.m., according to dispatch records.

They found a detached garage on fire, with a granny unit on the second floor.

The fire gutted the garage and caused smoke damage to the residential unit, Battalion Chief Jack Piccinini said.

He estimated damage at about $50,000.

Firefighters were told construction of the room had been under way during the day. The cause of the fire was believed to be related to the construction but remained under investigation Monday, Piccinini said.

Northfield Drive is near Piner and Marlow roads.