I would close up the window. You won't miss it.

I would remove window and soundproof space

I would have shutters installed on the windows and just keep that one closed

Close it up. You'll need lighting anyway - ambient, task and accent. LED's in 4000K daylight will be more helpful than that one window.

You cant leave it like that... you have to close it in

Anything you do will look weird ...cover it ..the windows are not balanced......paint it and preserve the light...if you cant live with that...replace both center windows with smaller matching windows

Or...paint it a funky color or use pattern to make it a conversation piece instead of an eyesore.. it’s an ADU, after all. I’ve seen a lot worse in basement apartments that rent for unbelievable sums!

symmetry

eta

OR...remove two center windows...center one bigger window ...leaving equal wall space on each side of centered “picture window”...

This could leave you with two extra windows ...if there is a spot for them...

Anywhere in the house...

Your window treatment can help with the plumbing window. As an ADU, the person living in the unit will want window treatments which will give privacy when the lights are turned on in that room. A combination of sheers for the daytime, and window shades behind the sheers for after dark may work. I would simply close up the plumbing window and hang a piece of art there. The two windows on the left and the three on the right could have matching sheers and shades.

Unfortunate example of "stuff happens". If finances allow, the solution of new windows and placement by @mdln...if not, remove window and close it up. While daylight is desired, any below ground space will need abundant artificial lighting. Having lived in several basement apartments the need for window coverings is minimal, the windows are SO low that you don't feel like you're "on display".

Another prospective You will hear water and u know moving through the pipe 🤷🏻‍♀️😁. Close and insulate! I have a unfinished basement that i set up as kitchen during remodel. Never noticed till i was down their everyday you hear all drains toilet flush

Thanks everyone for the helpful insights! Quite a few people made comments about noise from the pipe and to be honest I didn't even think of that. We will be adding another large window across from this room which should add quite a bit more day light to the area, so we probably can afford to lose this window. We were leaning toward closing it up and now after seeing all these comments, that's exactly what we'll be doing.

Under the circumstances mentioned...I would explore soundproofing materials for that spot as well