thank you __OIIIlO__,
It works :D
ATENTION: every 3rd building plan needs metal so this is how you do it:
1 - remove all residential buildins
2 - put one residential zone (requires metal, do not upgrade)
3 - put second residential zone (requires nails, upgrade it)
4 - put third residential zone (requires metal, do not upgrade it, YET)
5 - delete residential zone from step 3 (the one upgraded with nails)
6 - upgrade third residential zone (NOW upgrade it, the one from step 4)
7... - repeat steps 4,5,6 (every cycle gets you 700+ simoleons at a cost of 1 metal)
Here's a video I made with some great Trading Tips that will help you earn money too!
You can play the market and make money from the Global Trade HQ by trading against other players - and it doesn't use game loopholes which feel a bit more 'cheaty'
Hope it helps you out!