Fire crews tackled a blaze in a temporary building and a caravan in Hanham last night.
A series of 999 calls were received from people reporting the fire in a field off Abbots Road.
Crews used a high pressure hose reel to bring the fire under control, but the damage was extensive.
The building and caravan were not occupied at the time and Avon Fire & Rescue Service said it did not know if they were in full time use.
Investigators believe the fire was started deliberately.
