The Room of Hidden Things was apparently designed into the original plans. Students, teacher and house elves have been using it to hide stuff for over a millennium, presumably without realising that they're anywhere other than a convenient room to stash something in a hurry, which is of course how Harry found it.
He was standing in a room the size of a large cathedral, whose high
windows were sending shafts of light down upon what looked like a city
with towering walls, built of what Harry knew must be objects hidden
by generations of Hogwarts inhabitants. There were alleyways and roads
bordered by teetering piles of broken and damaged furniture, stowed
away, perhaps, to hide the evidence of mishandled magic, or else
hidden by castle-proud house-elves. There were thousands and thousands
of books, no doubt banned or graffitied or stolen. There were winged
catapults and Fanged Frisbees, some still with enough life in them to
hover half-heartedly over the mountains of other forbidden items;
there were chipped bottles of congealed potions, hats, jewels, cloaks;
there were what looked like dragon-egg shells, corked bottles whose
contents still shimmered evilly, several rusting swords and a heavy,
Assuming the room gets used just a few times a year to deposit one item, that would still be over 3000 separate items. If it's used occasionally to store more than one item, that could easily result in the room having literally tens of thousands of bits of bric-a-brac.