[Nehkara]

Also, the type of road you use is the upper limit for density.

Low density roads = low only.
Medium = low then medium.
High = low then medium then high.

Buildings upgrade in density (if they are adjacent to the appropriate road) when conditions have been met.

For Residential it is happiness that increases density (work, water, power, police, education, fire, health, parks).

For Commercial it is profit that increases density (although I didn't explore this as much).

For Industry it is profit AND demand for goods that increases density (again, I didn't explore this fully so I'm not 100% sure).