We have a tiny garden, no steps (save into the house) and a gate to stop DS going to the front garden and the road.

He's been playing out there alone... since he was about one and a half. Although at that point "alone" was I'd leave him outside if I needed the loo. We use the washing line and sheets to make a den, we have a ride-on and some cars as well as some water play; Liebling will be out there for hours if I wanted with minimal contact from me. So yeah, an hour to get housework done broken only by requests for more water, that works very well for me.

As far as I'm concerned, the garden is just another room in the house. I've always treated it as such so it has no special rules, other than you can get things messier in the garden. BUT we have a very small, child-friendly garden.