Around 25 firefighters remain at Alston Hall this morning more than 12 hours after a major blaze ripped through the historic building.
Fire investigators are now waiting for remaining pockets of fire to be extinguished and for building inspectors to carry out a structural assessment before they are able to enter the building, say fire services.
Firefighters are expected to remain at the scene for most of the day to damp down the fire and begin the investigation process.
Crews from across the county were called to the 19th century hall at around 10.45pm last night.
At its peak around 10 engines and around 70 firefighters worked to bring the blaze under control.
One person, who was in the property at the time of the incident, managed to escape the blaze but suffered an ankle injury, say fire services.
Damage to the Grade II listed hall is thought to be significant after the roof collapsed and much of the interior was gutted, although stone work may still be intact.
A spokesman for Lancashire Fire Services said: "When the first fire engines arrived at the scene, they found a large fire involving the three storey heritage building which is approximately 40 metres by 40 metres in size.
"The fire was affecting at least 30% of the building and firefighters requested 10 fire engines, two aerial ladder platforms and a command support unit to help bring the fire under control.
"Due to the amount of water needed to extinguish a fire of this size and lower water pressure, firefighters needed to pump water from a nearby pond.
"At 3am, 10 fire engines and two aerial ladder platforms remained at the scene and the fire service will be at the scene for most of the day.
"The fire investigation will begin later in the morning. One person who was at the property the fire broke out hurt their ankle before firefighters arrived at the scene but there were no further reports of injuries."
Alston Hall, designed by architect Alfred Darbyshire, was built between 1874-76 for colliery owner John Mercer.
It remained a family home until 1949 when the hall was sold to Preston Borough Council for use as a Day Continuation College.
In 1974, the hall was bought by Lancashire County Council who converted it to a residential training centre but was sold last year.
The house had a "little-altered appearance and interior" with "historic joinery, plasterwork, fireplaces and light fittings throughout", according to an Historic England listing which described the property as "highly decorative".
It is thought that the hall was being renovated by its new owners, local businessmen, when the blaze broke out.