Firefighters spent more than four hours tackling a blaze at the former Camelot theme park.
Crews had to pump water from the lake of a nearby hotel to stop the fire spreading to adjacent buildings.
Four appliances were called to the incident at Charnock Richard.
“It was quite a large fire when we arrived,” said watch manager Wayne Friend from Bamber Bridge.
“We struggled for a while. But once we got water from the lake we managed to get it under control.
“The building which was badly damaged looks like it was the offices and staff canteen. There was quite a bit of wood used in the construction of the buildings there and it burned very well.
“At this stage we don’t know how the fire started. That is under investigation.”
The blaze, late on Sunday, was the second at the site in just over two weeks. In the first incident a derelict petrol and oil storage unit was badly damaged in what was believed to be a deliberate attack.
Camelot closed for business in 2012 and plans for 400 houses on the site were refused last week.