The blaze caused "extensive damage" to Leslie House
It will take at least a year to repair the damage to a Category A building, which was being restored before it was overcome by a major blaze in Fife.
Leslie House, in Glenrothes, being renovated to create 17 luxury homes, was extensively damaged by a blaze, which started at 1900 GMT on Thursday.
Structural engineers are now securing the building ahead of investigators entering next week.
Workmen who were last at the site are being interviewed.
William Gray Muir, managing director of Sundial Properties, an Edinburgh company restoring the building, told BBC Scotland he felt very upset when he saw the 16th Century house in flames.
He said: "I was deeply depressed as I watched the fire but now I am determined that the house has another chance and that it will get back to as good as it was if not better.
"I have been working on it for four years and spent a long time researching the building and as I got to know it I got to love it, so it was very difficult to see the fire.
"However, there is now a lot of hope that it can be salvaged as the exterior is undamaged.
"We were about 18 months from completing all 17 homes there but now there will be at least another year added to that due to the fire."
He added that it did not appear that vandals were to blame for the blaze.
Bob Paterson, of Fife Fire and Rescue Service, who is leading the fire investigation, told BBC Scotland: "We are interviewing all the workers who were at the site before the fire in a bid to build up a time-line of events.
"Structural engineers are busy making the scaffolding structure safe so that by next week we should be able to enter the building to carry out a physical examination.
"It is a complex investigation because of the damage caused by the fire and therefore should take about two to three weeks to complete."