Firefighters from Bracknell station were alerted to the blaze after a motorist ran up to the Downshire Way premises and knocked on the door around 9.30am.
The driver of the Jaguar, believed to be in his 40s, was already out of the car with a young boy thought to be three-years-old.
Neither were injured.
The roundabout was partially-blocked by police while the fire crew spent 40 minutes extinguishing the flames.
It is thought the fire was caused by a mechanical fault.