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.
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Only handful of beaches have RNLI lifeguard cover during school holidays - the Maritime and Coastguard Agency warns.
Our Devon correspondent Seth Conway has been talking to a family who say without the hospitals, their children may not have survived.
It's going to be a windy one tonight and into the weekend - it could even touch gale force along the south coast.