The space clearance under the primary bed must provide enough room for the trundle bed and mattress. Depending on the manufacturer, the trundle frame can be as low as 6 inches. Add 6 inches for a mattress and the minimum height of the main bed is 12 inches. If a 6-inch box spring is placed under the mattress, the minimum height rises to 18 inches. Start the calculations with the collapsed height of the trundle bed frame and add the mattress and optional box spring. A couple of inches of clearance between the top of the mattress and the underside of the main bed allows for bedding.
Depending on how the trundle bed is used you may want the trundle bed to match the height of the main bed. Again, the specifications of the trundle bed frame are the starting point. Some frames rise as high as 15 inches. If a 6-inch mattress is placed on the frame the main bed should be no higher than 21 inches if the trundle and main bed are to match.
The trundle bed is removed from under the main bed for use. Leave the trundle at the lower level for two side-by-side single beds. Raise the trundle bed to the main bed level for a king size bed. The trundle bed can be moved anywhere in the room for use.
The main bed commonly has a foot and headboard while the trundle bed does not. Both the main bed and trundle fit single sized mattresses. The main bed can be configured as a bed or pillows and pads can be placed for its use as a couch.