I actually think it is quite fair that this teen has been allowed to have his own bedroom and bath in the basement rather than sharing a room with a younger sibling. I think before a decision is made on letting him have the larger room, that he would need to show that he can keep up the bedroom and bathroom that he already has. If he can show he can be more responsible in this area, then say you and your husband are willing to sit down with him and revisit the request.

In the meantime, you and Dad might go and check out the space to see how the smaller room might work for you to exercise in and as a play space for the younger siblings at times. He would certainly be the one to move and do anything else which might be required to make both spaces work. There should also be clear expectations set forth for behavior in the basement space with college age friends, especially given there are younger brother's around. Whether you want to write down the "house rules" and have him sign it or not, you may want to cover:

- Hours of guests in the space "hanging out" during the week if any and on weekends
- 24 hours house guests of any kind
- Noise
- Your stands on: drinking, drugs and/or smoking

A key to any decision would be continued suitable performance in college course work. This could work nicely in helping him have the life skills to move forward. Oh, and it is important if he is living at home that he have tasks assigned beyond his "basement hangout" which might incorporate: doing his own laundry, watching his siblings at times if you wanted a "date night", helping with yard work etc. If he is a responsible driver another option might be to help in a pinch driving younger brothers her or there (not regularly but in a pinch.) You need a balance of no free lunch, but also recognizing his need for some space, too.
In time as he moves on, the same approach can be used with a younger brother.

OP I think you need to rethink the MC decision. Maybe let him go away to college starting sophomore year. I think being away will do this kid a world of good and hopefully you can find a way to fund the extra year.

I'd cut the husband a break, here. Putting it on paper helps provide clarity, and his actions aren't the best. But we all know how families can get bogged down in less-than-healthy habits of relating to one another. Nobody is immune from that. Perhaps working through this issue on paper (internet) has been helpful to the OP and maybe give her a better idea which way to proceed.