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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Sunshine and showers today, cloudy with rain tomorrow.
Sun and showers today, cloudy with rain tomorrow.
A detective who helped bring the double murderer Christopher Haliwell to justice says he could also have killed Claudia Lawrence.