Why incomplete warships are difficult to finish later

Why not build a cruiser with spaces for weapons and complete it only when war begins?

The ship may be far from an appropriate yard, and fitting major guns later is far more complex than adding items such as armour; facilities and structural design matter.

The answer first asks where a ship will be when war begins and whether a suitable dockyard can perform the work. Adding armour can sometimes be comparatively straightforward, especially when it is attached externally, but installing major guns demands mounts, magazines, machinery, structural support, and a shipyard able to do the work. The County-class armour arrangements and Japanese cruisers built with six-inch guns but designed around later eight-inch weapons illustrate different kinds of planned growth. Nelson and Rodney then provide a machinery example: provision for extra boilers and shafts would have required deliberate design space from the outset. Together these cases show why a ship left short of a major capability may not be a quick wartime conversion.