+ Leo above. In simple terms, look on a Grant of Planning Permission as a Contract between you and the Council. The Council agrees to give you permission to build provided you comply with certain conditions. You agree to these conditions and start building.
If you fail to comply with a condition, then you break the Contract. A new Contract will be required but remember the Council do not have to enter into a new second contract with you.
As ONQ says, why don't you state what the impossible condition is?