Was dh out of order?

Why would OP’s sister have more of a claim on the room??

If DD was staying on her own, I’m not sure how it impacts her that much, as wouldn’t be sleeping under the desk.

I wouldn’t want to sleep under a bloody desk! What if you wake up disorientated and wallop your head!
He’s totally claimed that room for himself and I’d hazard a guess that your sister or in-laws won’t be staying very often now, if at all.

@RosesAndHellebores Our children will always have a bedroom at their family home. So they can come home whenever or if ever they wish

Well then you're lucky to live in such a large house that you can keep rooms available and pretty much unused for a lot of time.

Some people don't have a spare room (we don't) so people come to stay and you make do - living room, our bed and we move elsewhere, kids share, etc.

@autienotnaughty Having seen your DH's job, the desk definitely trumps the futon. I have three monitors in my office and could not use a desk smaller than the one I currently have.

I’d say you went outside our agreement and if the impact is dd won’t stay, then I will suggest she sleep in with me and you can experience your underdesk futon. That sounds fair, I’d be very upset if I thought dd wasn’t staying because of it.

It doesn’t sound workable to me. I wouldn’t like sleeping under the desk!

Id be afraid the mechanism would fail and it would come crashing down, or stuff would fall off if. What does he do with all his stuff btw?

@BringOle OP says it can be converted to standing up desk and a futon is low down.

So you freely admit he needs this size desk and it's more comfortable for him, the person using it the majority of the time, and yet still think be was wrong?

My mother and step took over my bedroom when I was 21 and had moved out. It was visceral. My father would never have done it. I knew precisely where I stood!

What need did you have for a bedroom at your parents house after you moved out? It’s your parents house to use as they please. Why keep a room empty and unused if they have need for the space?

We lived in a 3 bed house where the smallest room is a box room.

I fully expected it to become an office when I moved out

We didn't have room for me to lay claim to a room I no longer used

I would never have taken it as a show of territory marking. That sounds like you have some hang ups about your mother remarrying

If your DD took her bed, and a "spare king size," where was that spare bed before? Why can't that be the guest room?

that’s a very high desk (or very low futon?) if a person can lie down on the futon but still be under the desk! Sounds like a sardine tin. Personally I couldn’t

how about getting a pull out armchair bed? Swyft have nice and comfy ones

You shouldn’t really ever need to put monitors forward or back, they should always be about an arms distance away from you when sat at a desk.

It doesn't help the situation, but I would have thought it more useful in that situation to have two desks and a swivel chair, otherwise you're constantly pushing your monitors forward and back.

Does his job need him to have a large desk? YANBU to ask him to send it back/sell it if it's something he doesn't need for work and not on the list of ones you agreed on.

Reminds me of when we moved into our first house and MIL gifted DH one of those enormous corner desks like they use in call centres. He thought it was great, but it took up 3/4 of our spare box room! He didn't need a desk that big, could have had a smaller one and put up some shelves to store his stuff on. It was so bulky you couldn't reach to clean the widows properly and they got mouldy .

Edited as I've just seen your update that a bigger desk is more practical for him. Could he have a smaller desk for his computers and use a fold out table for working on, so you can pack it away when you have guests?

his monitors are at the back and then he has room to write/ draw (he’s a engineer)

@Greenvases he has probably done it as practical for work. In first few weeks of lockdown DH WFH on the kitchen table, nightmare with his big screen. Was much better when we sorted out desk in spare bedroom.

The DD doesn’t sound too hard done by as took her own bedroom furniture plus an additional bed to her new home. What was she expecting OP to do in her old room?

It's a difficult one, but on reflection I think I'm on your DH's side here. If he's an engineer and needs room on his desk for drawing then a corner desk just isn't going to do the job. He's probably been dreaming about the day he gets a proper desk rather than the kitchen table.

Plus it has the added bonus of perhaps preventing your sister from coming to stay for 1-2 months every year! Sorry, I know I'm projecting now but I would have hated having my SIL stay for that much time. I'm sure your sister is lovely though.

What does he do that he needs a desk that large?

Yes and I noted that information as an exception. It's a shame the girls step is not navigating a middle way imo.

So you'd rather your DH was uncomfortable at a desk for 52 weeks of the year, less holidays, for something that maybe would be 8 weeks?

Your DD didn't even stay there much before she moved out

She means on a futon. As she said in her OP

Why wasn't it sent back?
He did this deliberately.
Making guests visiting more difficult, especially your child.

I would be very pissed off and it wouldn't be staying.
HE can buy another one.
He sounds like a selfish, dim, twat.
He would not get away with this.

@RosesAndHellebores can you at least accept that not everyone is in the same financial position as you and won’t be able to keep the family home (together with a shrine for their pensionable age adult DC) until they die

Gosh you really are over extending now.
He does what he needs to do and goes over and above compared to many others, with his mother's wishes and best interests at heart.