Cornwall Fire and Rescue Service said 11 residents had to be evacuated from the property in Eastborne Road.
Six fire engines from St Austell Mevagissey, Fowey St Dennis and Lostwithiel tackled the blaze.
The building is used as sheltered accommodation.
More top news
The whole region staying clear overnight with warm temperatures.
Temperatures remain high up to 12C inland.
Until now the ground has just been too waterlogged to let them out to graze.