I had double glazing windows installed. No need in my case to go outside the building, all work done inside. Five windows including 2 one meter x one meter squares. The old alum frames are removed, or not, and new glass is popped in and all sealed up. One day of work but of course had to contact shop to measure, and chose type of glass, one week prior.
Cannot recall precise cost but i thought not too expensive. No need for me to contact anyone about my wants as i own the entire 2nd floor. And (full disclosure) i also made no contact to govt agent as to my plans.
A big improvement in noise reduction so i'm pretty happy. The glass i chose was translucent not clear since outside the window, about 15 feet away, was someone else's flat. Now the light beams in but not the street chatter.