Why carrier armour had to be designed in from the start

Did 1930s carrier designers plan to add flight-deck armour later?

Some concepts considered later armour, but placing that weight high in the ship risked invalidating the carrier's stability calculations.

At least one 1930s concept considered adding flight-deck armour later, but it was never built. The armour sits high above the waterline, where added weight has an especially strong effect on stability. A retrofit could upset calculations made for the original unarmoured ship. Designers therefore had good reason to settle the armour scheme when they first defined the hull and its weight distribution.