Anthony Perry's work on the Church of St Andrew in Chichester was thrown out by councillors late last month - despite being praised by two former bosses of the Royal Fine Art Commission.
Both Sherban Cantacuzino and the late Professor Freddie Fielden had lent their support to the project.
But local politicians at Chichester District Council's area development committee rejected the application and English Heritage's David Brock labelled Perry's supporters 'too exalted'.
In addition, Perry's proposed Modernist addition - the octagonal structure pictured - was deemed by the council to be out of context with the original 12-14th-century building and a 1981 extension.
Perry said: 'There's a very strong tendency in Chichester to continue the neo-vernacular style that is liked locally. We're trying to be very clear about what we're doing.'
But Brock attempted to justify EH's objections.'I do think doing something Modern in this location would be doing something wrong. I would have liked to have seen what we could have got out of another gable,' he said.
However the founder of the Chichester Society David Goodman said of the plan: 'I think it's a charming addition. It's strongly approved of by those who know what they are talking about.'