Surely, Sen, planning permission shouldn't be a problem if you're replacing an existing structure by one of the same size and character? Although the new one would have to comply with today's more stringent building regs. If all else fails, dismantle & reassemble on a new base, replacing original bits where necessary, and call it a repair - AFAIK you don't need planning permission for that.

Admittedly the last time I had anything to do with planning permission it was for a garden shed only just tall enough to need it. A nice man at Aylesbury Vale DC said make the new concrete base 3 inches lower, then it wouldn't be.