For warships, that always seems to be problematic because they always want to add stuff on above the waterline.
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Engineering Change Control Boards are supposed to prevent that from happening. Everything that’s added or changed in some way is evaluated for potential impact.
LOL, absolutely, but even we had issues with that in WWII, I am guessing mostly due to wartime contingencies, and we had a dedicated part of the USN (Bureau of Ship Design BuShips I think) that tried to prevent it.
One of the big issues was as the war progressed, they realized they needed far more AA weapons on warships than existed, particularly in 1942-43, so they developed a tub/gun system that could be added, and the addition of those all over the fleet did make some ships marginally (or actually) unstable, especially older ones built before the war.
I read a book recently called “Warship Builders: An Industrial History of US Naval Shipbuilding 1922-1945. Much of it was boring to me dealing with bureaucratic infighting and such with the makeup of the administrative arms of government/shipbuilders, but...the way that we upgraded shipyards with government grants, the equipment they maintained, and the actual shipbuilding processes (especially the Iowa Class BBs) was just fascinating.
In this case of the Thai navy, I don’t know. The ships were probably designed with margin built in, but who knows what was added afterwards, or what process it went through to add them.
These AA tub/gun things they designed were made so that anywhere they could find space (or build it on a sponson or something) they could simply lay out the area in chalk on the deck and weld the tub gun assembly into that if space allowed.
I got the distinct impression that they decided early on that the risk of being bombed was greater than the risk of capsizing!