Since the purpose of family vaults was to house multiple generations, underground storage became a necessity. So how deep do vaults go? Several meters, sometimes with more space below than there is above.
If the mausoleum occupies a wide enough lot, there will be a staircase which leads down to lower levels of storage. Narrow lots may not have room for a staircase, but remove the metal grate & ladders accomplish the same goal. It makes for a tight fit in some cases: