Integrated worlds, separate, or co-existing?

[Sandra Collins]

No: HPFGUIDX 154841

I don't know if this has been discussed before but I'll give it a go. 
Over the last few months I've been through my HP collection 
again due to a poor performance at a HP quiznight at a friend's 
house, starting from book one and going right through. I just 
can't get my head around the layout of the whole HP world.
If a non wizard/witch person was driving around, could they 
stumble across the Weasley's house? Would a burglar be in for 
a few surprises if they chose Snape's place to break into?
I suppose my point is, if the characters have to go through a 
magical gateway to be on Platform 9 and 3/4 which exists in a 
'parallel' world, why don't the magical people reside permanently 
in that world? I can understand that those children with one 
parent who is magical and one who isn't magical might need to 
go through a gateway, but why would the Weasleys (and 
presumably the Malfoys as well)? I first wondered this in book 
two, when Ron is flying the car in the non-magical world, and 
somehow arrives in the magical one, flying above the Hogwarts 
Express. I think the same is true of Diagon Alley as well. I think 
that is in a 'parallel' world as well and takes up no space in 
London otherwise it would have been seen over the years and 
the shops would be having a hard time with the businees rates!
There's a few other questions this raises, but those are for 
another topic and won't be important if someone can put me 
straight on things I must have missed.

Sandra, back after ages away!