The new Oak Park Hospital in Havant may not be equipped enough to handle the growing numbers of patients, slated to be heavy in the next 20 years.
Hampshire County Council's health watch dog, concerned with the capacity of the hospital, has demanded a detailed report.
Havant Councillor Ann Buckley, a committee member of the health watch dog, said: 'We know bed numbers have fallen and people do not stay in hospital as they used to. We can understand that. But we feel we have not been given the background information to give us the confidence that these are the right numbers.'
Inger Hebden, director of capital planning at Hampshire PCT, said: 'The key is to make sure we build community hospitals that can last 30 years, hopefully 60 years, in such a way that they can cope with changing needs. We are keeping some of the adjacent land in PCT ownership in case we need to expand.'