Merseyside Fire & Rescue managed to get the blaze under control after three hours.
The brigade said the fire involved an external gas main that spread to three houses under construction.
Two people were taken to hospital with minor injuries following the incident at the Pavilion Gardens development in the town.
A David Wilson Homes North West spokesperson told the Liverpool Echo: “There was a small fire in an unoccupied building under construction at our Pavilion Gardens development in Southport today.
“We would like to thank the emergency services for their swift response in making the site safe.”