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?
You can either place high density roads, but if you already have a road network in place, you can just upgrade what you have.
To upgrade a road, click on the roads menu and look for the Road Upgrade button at the bottom.
Click the Road Upgrade button, and click on one of your roads to get the upgrade menu. The upgrade has a cost of course, based on the length of your road.
You can’t just create high density roads though and expect high density buildings, you have to keep your wealthier residents and business owners happy.
Click the little smiley button at the bottom of your screen, and you’ll bring up the Approval Rating screen.
This is an important screen to keep an eye on. The sims in your city will tell what they enjoy, and what they hate. If you can keep them happy, then they are more likely to to upgrade their buildings eventually.
Obviously each zone type has different wants and needs. Residents want high paying jobs, clean air, and education. Commerce wants happy, high paying customers. Industry wants good workers.