This is the Primitive Methodist Chapel at High Street (St Austell) SW965533.
The chapel is marked on early maps (1881), becoming a Free United Methodist Church by 1907, and labelled simply as Methodist Church by 1932.
It was disused by the time of the 2nd world war when American troops camped nearby and is referenced in the BBC War Stories collection.
Planning permission was sought in 2004 to change use and this was approved to one bedroom holiday letting unit and home office. But this does not appear to have progressed much.