Flames could be seen high above the homes on Viewmount Crescent, near Mayfield Road and Hurontario Street, at 5 a.m.
Paul McCormack lives next door to the property where the fire broke out.
"The heat here was really oppressive," said McCormack, whose trailer was damaged by the flames coming out of the houses on Thursday morning.
Firefighters managed to keep the fire from spreading to other houses.
No one was injured.
The charred homes had been bricked and roofed. Families were set to move in later this year.
Fire Chief Andy MacDonald said there are no obvious accidental or natural causes to the fire and it is considered suspicious.
"We're treating it as a potential crime scene," MacDonald said Thursday.
Police and the fire marshal are investigating.