I didn't dig & fill the whole 3ft though haha. I just dug the 'countours' for the concrete foundation and the wall (so it was like a bare mount when we began). See pic here for clarification
This week I just dug out about a foot along the whole surface are because it was too high up and if I wanted to amend it to a reasonable debt to prevent waterlogging, I'd have no spare room (volume/height) to add anything, it'd be higher than the wall itself then. Now I can just top it off again with GOOD 'soil' (sand, gravel, compost) and mix it in as deep as I can
As far as a tree goes...I'm now very reluctant to plant one. Yesterday I put the hose between the two back walls (so our new one and the neighbours' kitchen one) and poured water in between to settle soil behind it. 10 mins later my neighbours comes out announcing her kitchen's under (muddy) water.
So somwhere there's a non waterproof (so non root proof) leak or hole in her wall. I immediately stopped ofcourse and sealed the gap between the walls with waterproof concrete. So that's that leak sorted out (hopefully), but there could be more deeper down. Not that it's my problem, but I don't want to be accused when some years from now there are roots poking out behind her cabinets or her floor is showing cracks or whatever :S
