Firefighters responded to a report of a structure fire at 1739 W. Third St. at about 9:40 p.m., according to the Santa Rosa Fire Department.
Arriving firefighters found part of a four-unit, two-story townhouse engulfed in flames. The fire was burning in one of the middle units and spreading to adjoining units on either side.
The fire was controlled in about 40 minutes. No one was injured.
All four units in the townhouse were vacant at the time of the fire. The entire structure had recently been deemed unfit for occupancy due to building, health and safety code violations.
The fire is considered suspicious and is being investigated by both the fire and police departments.
The fire caused an estimated $100,000 in damage.