It depends on the context:
* As a noun: It's often used as a noun, referring to the northern part of a city or region. In this case, it's usually written as two words: "the north side of the city."
* As an adjective: It can also be used as an adjective, describing something located on the north side. In this case, it's often hyphenated: "the north-side neighborhood."
So, "north side" can be written as two words, hyphenated, or combined as one word depending on how it's used.