Certain child restraints require the use of a support leg that rests on the vehicle floor.
Some vehicles for example people carriers may have underfloor storage boxes. If you have underfloor storage (and thinking of purchasing a seat with a support leg) consult the vehicle's handbook or speak to the vehicle manufacturer directly to get clarity on what is approved when using a child restraint support leg on a storage lid.
The mass of conflicting information available can certainly cause confusion and compromise safety. The issue with the support leg and (empty) storage box is that in the event of a collision the support leg could break through the storage lid. The support leg requires a solid stable surface such as the chassis of the vehicle. You may have the option for the support leg to reach the bottom of the storage box (if it reaches and that is approved). Again clarify with the vehicle handbook or the vehicle manufacturer.
Additional Supporting Information:
Data varies by the vehicle manufacturer (and child seat manufacturer fitting lists). Not enough data and tests have been carried out on underfloor storage systems. Some vehicle manufacturers offer ‘approved fillers’ for the storage box. You may also come across contradictory information about what an ‘approved filler’ is.
If you have a seat with a support leg fitted on top of an empty storage box. Is it possible to move the child seat to the middle position potentially eliminating a storage box? Always check if the seat is approved for its position and if the vehicle handbook advises of any restrictions.