SimCity (2013): How to Increase Density
In SimCity (the 2013 version, aka SimCity 5) on the PC, unlike in SimCity 4, there are no longer set zone densities. The density that a zone becomes is now dependent on other factors.
The primary factor is road density. If you want a high density zone, you need to have a high density street. Like in the real world, you won't likely have a sky scraper on a residential road right?