Easy. "SS, that's what your dad and I decided to do. When you have your own house you can do whatever you want with the rooms."

Any suggestions on how to handle this crap show??

I don't know, laugh? I mean, I wouldn't even entertain letting a kid have the living room of my house to themselves.

Ask him how he will pay for it and that until he can fully participate in the budget of the house, he can pound sand to find a job.  Twirp!   He needs a dose of reality and delivered in a way only a snarky, fed up stepmom can deliver.  F them all in your house! 

I find it laughable that the little twerp thinks he gets a say in a home where he pays no bills and his name is not on the deed.  Daddy needs to put Junior in his place and tell him it's for the grownups to decide and when he gets his own place he'll have his own living room.  

Well, of COURSE you don't understand why I need a study, you're just a child, SS! Maybe, when you're a grown up, you can have your very own living room, when you have your own home. "

OSD wanted to "help" me decorate the house when we moved in. Or she wanted to re-configure the kitchen. I told her "When you have your own place you can do that all you want." or "Sorry, you dad and I own it, you have your room, and that's where it ends at this point." Because at BMs she DID get to do all that stuff!  DH hated it when I used to say that I paid the mortgage and bills and that's where it ended - but HE also started doing that too once he was fed up! LOL.

The issue is that this kid has never been shown his place in the relative blended family universe.

He gets no say on anything regarding the home that you and his father provide.  If... you ask for his opinion then he gets to express his opinion. Otherwise, he gets no opinion.

This is it right in a nutshell. Rags smashes it out of the park once again.

"This kid has never been shown his place in the relative blended family universe"

100 percent this!

OP, simply say to this upstart that when you want his opinion you'll ask for it. Also, as other posters have said tell the little smartmouth that when HE pays the bills then he can decide room configuration. 

Until then, butt out.

it probaly wouldnt matter whether you said anything in front of him or not,,,,imagine setting it up for your own space and he would just take it over anyway by the sounds of it.  he has his own room , that all he gets when hes not paying the bills ...end of story!

I would tell him that it is humorous that he thinks he can demand a second room in a house he doesn't pay for or contribute to financially and that he feels entitled and "owed" an explanation for any decision made in someone else's house. Bust out your phone the next time he's ranting about the study and tell him you're saving it for when he's a grown up and you and DH visit him for holiday and he doesn't give y'all your own room to stay in for the visit. 

"Sure SS, infact you can have the office I was planning on. Rent for your 2 rooms will be $2,000 per month. Let me know if you want to pay with check or Venmo. Since you have those 2 rooms, the only shared room will be the kitchen."

Funny the boy who pays nothing thinks he should be the one to decide where everyone should be lol

My SS asked for his own bathroom, a study for himself (when he doesn't study) in the past as well. DH shut that down real quick, and I hope the next time we move he'll be on his own (i really hope).