Arsonists torched an empty Burnley house on Christmas Day.
Fire swept through a house, which was under construction, on the Sycamore Park development off Gannow Lane.
When fire crews arrived at 18-45pm the blaze had spread through the first floor of the vacant property.
Firefighters wearing breathing apparatus entered the building an managed to bring the flames under control.
Watch manager Steve Harrison, of Burnley Fire Station, said: “This kind of fire within houses under construction is particularly dangerous for fire crews due to the nature of the interior, no staircase or floors may be missing and fire can spread rapidly.
“These kinds of incidents are problematic for the fire service to deal with, but they also have the potential to cause injury to anyone who may enter with the intent to start a fire.
“The missing staircases, difficult access and rapid fire spread could quite easily cut off escape for anyone inside.”
He added: “Do not be tempted to play within building sites, and most definitely do not start fires within them.”
The blaze is being treated as suspicious and a joint investigation into the blaze has been launched with the police.