Does a booster seat HAVE to be installed on a seat?

[Slappy]

Just wondering because I'm thinking about doing up a box for storage and stuff to replace the back seat, but rarely need to be able to carry my kids in the back. Car has had anchors installed professionally, if I just kept seat belts, clips etc in there would that still be suitable?

Can't find laws on this specifically, I can't see why it wouldn't be ok, perhaps a little bumpier but the restraints etc are all still there.

 

[BarneyB]

The laws are not appropriate for my family.
As soon as you meet my kids you're thinking, "they should ride in the tub or on the roof racks".