I play retail WoW regularly, but I’m ready to try Classic. The launcher needs to install and asked for an install directory.
The default location is C:\Program Files\World of Warcraft, but that’s where retail is installed right now and I don’t want to overwrite it. Is Classic supposed to be installed in the same directory as retail or in its own directory?
No need to worry, the launcher creates a Classic folder where all relevant data is placed. Your retail folder will be safe.
Thank you, Sedai 
I was surprised how little extra space is used. Classic and Retail use the same engine. So here’s how the 3 sub-directories break down on my PC:
Classic = 193 MB (less than 1 GB)
Retail = 184 MB (less than 1 GB)
Data = 63.1 GB
Launcher = 4 MB
Total (all of WoW) - 63.5 GB
Before I installed Classic, my Retail was somewhere around 55-60 GB. So Classic didn’t double the size.
Ya, I was surprised at how small classic was as well. Even standalone without retail.
Just a little addendum - if you plan to use addons at all, make sure you’re picking up ones written for WOW Classic and that you put them in _classic_\interface\AddOns.
If you use the twitch app it does that automatically, there is a drop down menu on the app for retail/classic, just be sure to choose the right one, the addons are updated separately too.
Classic actually uses the data files from Retail. So the directory will literally be the same. It will store addons and UI configs separately however.
I remember my first time on a computer too…