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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It will be patchy but some heavy bursts wrapped up in it. By the early hours its across most parts.
More rain will only top up already saturated ground. Thats what is going to happen tonight.
Dry start but unsettled midweek, colder by Friday.