New Basement Darkroom Advice

As OP will be washing prints in the next room, there won't be that much time spent in the darkroom between print washes. There will be some air exchange with every entrance and exit.

The chemicals we use have an odour. The air coming out of the carbon filter won't have an odour so no chemicals to breathe in. Best solution is to vent outside of course but the carbon filter is workable stopgap solution imo.

My darkroom isn't much bigger than the OP's. I have a fan but don't use it all that much.

You can easily frame in a space; especially in a corner. Use sheetrock, chipboard, or cheap paneling on top of the frame. The ceiling always seems to be the most difficult. You can try to caulk every crack or staple in ceiling tiles.

Getting water to your new darkroom is relatively easy. Draining it is the hard part. If your sink is just on the other side of a sheetrock wall, you might be able to tie into that drain. Or if you have to run a hose to a floor drain in that room, it may work.

Since it looks like you can cut the lights in the basement as a whole, a curtain may be all you need for a door.

Code in MN allows you to do electrical work in your own home (assuming not a rental). But to do electrical work in someone else's home you need to be licensed. Other than the electrical, the biggest other code issue might be anti-siphon devices on hoses (easy to add).

A MUST - make sure you have GFCIs on your outlets.. Generally easy to do by simply adding a GFCI receptacle in the right location.

My advice is don't obsess like I did. A few tiny light leaks from the distance won't affect anything. I built my darkroom like Fort Knox and probably wasted six months obsessing instead of shooting and printing.