An Extradimensional Hold
The hold of holding is actually a knocker that one bolts on a door. Once bolted onto the door, the number of knocks given along with the command word spoken the hold of holding enchants the door to become a portal to an extradimensional space.
The size of the room depends on the power of the enchantment, and each tonnage is equal to 100 cubic yards.
Tonnage Minimum of Cargo Area Caster Level Market Price
1 ton 20 ft. by 15 ft./10 ft. height 1st level 14,000 gp
2 tons 20 ft. by 30 ft./10 ft. height 2nd level 28,000 gp
3 tons 40 ft. by 25 ft./10 ft. height 3rd level 42,000 gp
5 tons 50 ft. by 30 ft./10 ft. height 5th level 70,000 gp
7 tons 50 ft. by 40 ft./10 ft. height 7th level 98,000 gp
9 tons 50 ft. by 50 ft./10 ft. height 9th level 126,000 gp
While all the holds listed here take a minimum spellcaster of 13th level some really large holds would take much more.
Caster Level: 13th; Prerequisite: Craft Wonderous Item, mordenkainen’s magnificent mansion, creator’s class level must be a equal to the tonnage of hold; Weight: 5 lbs.