Do I need permission and permits?

With the market fairly stagnant and mortgage rates low, instead of trading up to a better property, many unit owners  are making more of what they've got. 

But is it as easy as it sounds? Do you need permission and what kind of permission from whom? 

Do you need an Owner Builder Permit and, if so, how do you get one?

I'll try and dig up some answers but meanwhile feel free to chip in with your thoughts.