Cribs are required where I'm at, but DD slept on a Pack 'n' Play mat on the floor as soon as we had a bedroom for her (around 6 mos). She rolls out of bed--still does at 6 years-- so we never used a toddler bed at all; just put the crib mattress on the floor, then went to a twin on the floor when she outgrew the crib mattress. We finally bought a bed frame a few months ago, but she keeps the spare mattress pulled out from the trundle space because, yep, she falls out in the middle of the night.
If you're doing a floor bed for your own child, you just have to be aware of safety hazards and thoroughly baby-proof the entire space, so it basically becomes one big crib. It's more work but it allows the children more space to develop their gross motor skills and self-soothing skills.