Being that the custom sewing is your job/business, then it's also part of that business for you to finish the room that was intended for it. Just.Do.It!
Yes, there is another problem ... it has been used for sometime for other than for your business. And it appears that it's been for more than storage ... you mentioned a flat screen, and his watching TV. Looks like he has changed this into his man-cave.
Take hold ... and start emptying the stuff, and converting it back to your Business Location!