No, it's not. The 30m2 threshold is the limit to build without building regulations. There is no size limit for permitted development except the area of the curtilage developed, and the fact that an outbuilding must be "ancillary".
In any case, why permitted development? you are going to really struggle with that area, because by the time you are out the ground, and allowing for roof timbers and insulation, there isn't much left. Who wants 2.1m ceilings?
what you have drawn is way over 2.5, unless those are special doors for little people...
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