The arson squad is investing two suspicious fires that destroyed homes in the region in the past 24 hours.
Firefighters were called to a blaze inside a unit on Uldina Place in Pinjarra around 5am on Thursday morning.
The three occupants of the home were safely evacuated, but the roof of the unit suffered extensive damage.
Officers are treating the fire as suspicious and a 42-year-old man is assisting police with their inquiries.
It comes less than 24 hours after firefighters battled a blaze in Mandurah on Wednesday afternoon.
Emergency services were called to the home at the intersection of Aldgate and France Streets around 4pm.
Investigations into the suspicious fire that destroyed the home are ongoing.
Just last week, a home on Ammerdown Place in Erskine was destroyed by another suspicious blaze.
It's believed the fire caused an estimated $400,000 in damage.
Anyone with information or CCTV footage of the Mandurah, Erksine, or PInjarra areas around the time of the fires is being urged to contact Crime Stoppers on 1800 333 000.
State Government to fund Coodanup boat ramp upgrade
Locals claim $1m Lotto wins
Double demerits now in force over Christmas period
Firefighters contain and control Furnissdale blaze
Baldivis named as worst car park crash suburb in WA
Mandurah man nearly misses out on $150k Lotto win
Port Kennedy bushfire deliberately lit
Emergency crews kept busy with Dawesville boat fire, Greenfields house fire
Lotto wins for Halls Head and Byford