Why the Admiral class could not simply become fast battleships

The Admiral class began too far from the desired fast-battleship design for changes during construction to complete the transition. The three ships after Hood would have gained more subdivision and moved closer to Tiger's approach, but they would still have been battlecruisers in this classification. Their uncertain future included consideration of aircraft-carrier conversion before cancellation. G3 stood much closer to the fast-battleship boundary. Some additional armour coverage lower on the hull and further division of internal compartments would probably have moved it across the line in this assessment. The distinction concerns the extent of protection and the structure behind it as well as belt thickness.