Mississauga requires a permit to enlarge a window. Doesn't matter whether it's vertical or horizontal height.I might be wrong but I do not think a permit is required for replacing windows. Especially if you are not doing anything structural to enlarge the windows cavity or making it smaller.
High voltage line may not need to be removed but the Utility company (or its contractor) may install the orange covering you often see on reno projects.
Toronto used to not require a permit if you're just going down, but did require it for horizontal enlargement. Now they require it for both.
I called the City and confirmed. I'd do it without, but neighbour will 100% see it and call in. I only want to do 1ft down to add more light. But if I need a permit, I may as well expand them horizontally also to just add a ton more light. While I'm at it, I'll get a permit to expand my son's window as it's very dark, and the MIL's window will become a door to the yard.
Now just need to find my dad's $300 laser measuring device as he's going to lose it if I tell him I lost it. Need to measure out the house properly.