lucyk, I've let my ds go around the house on his own for ages. When we moved to our current house he was 3 y 5 mo and I was happy for him to be on his own for as long as he liked but he never seemed to want to be far away from me for very long then. We do have stairs but he was completely ok on them, all plug sockets have covers etc, windows were shut and couldn't be opened, and nothing terrible ever happened. I would be able to hear him if I was at the top of the house but only if he shouted loudly enough!
Now he's nearly 5 he's certainly allowed on his own all over the house. In fact, if he wasn't there is no way we would be able to get ready for work in the morning (he's given breakfast, gets dressed and then has to amuse himself for half an hour while we get ready). He knows about not touching the kettle, oven etc, he knows how to turn the telly on, but not how to get the channel he wants (note to self: must work on that one!) and he sometimes seems very happy playing on his own as long as he knows where we are. OK, he very occasionally does something he shouldn't, like drag a chair over to the cupboard to stand on to get any left over cakes he thinks might be on the top shelf but hey, he's careful and I admire his resourcefulness! Mmm, must put them lower down so he doesn't need the chair though...
He still won't be on his own for that long, just because he wants to know where we are so will eventually come and find me/us if we're not in the same room but our house is safe for his age and for us, this feels right. HTH.