Permitted development/planning permission

[Sophiathe2nd]

Anyone know of you can get permitted development for a loft and then add more to that loft under planning permission is this a way round the system?

No, is the short answer.

All the allowances under Permitted Development are measured from the building as it was originally built (or as it stood in July 1948, which is when the modern system of Planning legislation came into force).

You can't extend, then extend the extension, as a way around the limits.