Quote from @JD Martin:
I'm still trying to picture in my head how combining a 2 unit into a 1 unit makes you lose light. Essentially making 2 into one is mostly a situation of just taking down dividing walls, unless you were changing interior partition walls which probably required a permit then so why not just do it then?
In any case, well that depends on your risk tolerance level. If you know what you're doing adding an exterior window is generally child's play in construction terms, especially if your walls are opened up and they're framed instead of masonry. If you don't know what you're doing you could seriously compromise your roof and the framing structure in that area, which is why you generally need a permit. You also generally have restrictions & qualifications depending on where the window is - if it's very high up from the ground, you might have to use a certain glass or the window might have to be a certain height. If it's in a basement area there may be size requirements for egress or light. ETC.
What you are risking can't be answered because it depends on where you are. Some places will make you rip everything out, fine you, and make you start over. Some places won't care unless someone complains. Some places won't care at all. As far as risk management, if you did this and didn't know what you were doing and someone got hurt later, and sued you, once your insurance company found out you did it without permits they wouldn't defend you.
Thank you for your answer JD.
1. when i merged the 2 units, i walled one of the entrances and its staircase to create more floor space. That blocked the light coming from that side which had small windows.
2. i didn’t ask for a permit for the merging, which is also another issue as now if the city inspector would come, they would be asking me about the other work… this part is ok as i am planning to revert it back to duplex when i leave to rent it out. But meanwhile i would like to have more light while i live in it.
3. i don’t mind paying professional good contractors who would do it the right way. It’s dealing with the city that i am trying to avoid as they have long delays and they charge a fortune for these things and because of what I said in 2, they might deny it.