Some rationale must apply though, if it ends up looking radically different then the approved drawings have not been followed and arguably it is in breach.
Planning permissions usually state: "must be be built strictly in accordance with the approved plans blah blah blah...." or words to that effect.
So imo the answer would be 'no', it would require a new application.
But of course in the real world you're right. The council won't do anything.
Technically the OP could say it's work in progress and will be completed when he's got sufficient funds!
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