Juku I relate to this very much. My a would fill a room with junk, then move on to the next and constantly complain that "he had no space in this house for himself!" which was ridiculous as he had filled each and every inch of space with his junk. Boxes and cases and bags of his "stuff". There was not a single place in the house not filled with his garbage; I hated it!! I once had a look in some of the boxes that were filling our dining room. One of them contained only a solitary old holey sneaker, a broken electrical cord to nothing and a dead spider. Each and every box and bag was like this...just mountains and mountains of literal garbage that needed to go onto a bonfire.
It was very hard to address this with him as he too would have a hissy fit "you don't want me to have ANY space in this house" he would cry... but in the end I gave him the option...choose which rooms you want and which are mine to use and stood my ground. I did have to do the entirety of the work myself moving all of his "belongings" to his space and then making mine my own but it was worth it and I rejoiced in my own space when it was done. And believe me it was a monster of a job, garage and all....UGH.
Hugs and affirmations from me...you can do it!

Hi Jules. You're a creative. Creatives are messy people. We surround ourselves with half-finished projects, tools and materials that we envision as future works of art, aesthetic or functional improvements, or maybe just expressions of ourselves. Healthy creatives just clean up every once in a while. He's a creative too, or used to be.

My experience with my A is that I always thought we would make decisions together as partners about our surroundings, but while I entered into this with the mindset of "nothing is done until we agree", she does not have that mindset and has always done things however she wants regardless of my objections or input.

WARNING - POSSIBLE BAD ADVICE TO FOLLOW

As difficult as it was, I finally started doing the same thing (after 15 years). I don't do things spitefully, just to get back at her (I did a few early on), but if I want something handled a certain way, I just do it. I've found that, there are a few things that we just silently battle back and forth about (how the kitchen is arranged, how the laundry is managed), but I do it my way and she does it hers; but I've also found that, on some things, she eventually gives up, as I used to do. She doesn't want to discuss it because she knows she doesn't have a leg to stand on.

So what I'm getting at is if you want the studio, just take it. If he says anything just say "I'm making a studio" like a stone-faced robot. He'll move his stuff in, and you'll move it out, and vice-versa, just don't give him the fight he's looking for. J.A.D.E. - Don't Justify, Argue, Defend or Explain, just say "I'm making a studio" over and over, and he might eventually give up. As usual it's more work than you feel like you should have to do, and you're right, you shouldn't have to. "should, shouldn't, would have, could have" doesn't change anything. If you want it for yourself, you have to get it for yourself. When you live with an A, they just make it a little harder (and yes, sometimes impossible).

Anyway, don't know if it'll work, but it has worked in part for me.