Why heavy turrets are difficult to add later

Could a ship be built fitted for, but not with, a triple heavy-gun turret?

That is judged impractical: the turret and its supporting installation are too large and costly to treat as a simple later addition.

A heavy turret is only part of the installation that would have to be added. The assembly, magazines, and other supporting arrangements would also need construction or alteration. That makes delayed fitting exceptionally expensive. The reply distinguishes this from reserving provision for light guns or missile systems that can be installed more straightforwardly.