Demolish

I have applied for permitted development approval to build a brick sun lounge, in the place of an existing conservatory.
The new sun lounge meets all of the criteria set out in the act as a Class A permitted development, however the Local Authority are insisting that planning permission is sought.
Their basis for this is an interpretation that I am replacing / rebuilding an existing building and the application then falls outside of the permitted developmet regulations.
Can anyone advise me if this is correct?

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