Am I being unfair expecting OH to actually parent his eldest?

Sorry you think you’ll get homeless accommodation because a 14 yo is pissed off his dad has created an overcrowding issue and isn’t stopping younger siblings breaking his stuff? You know if op gets anything it’ll be a b & b or a premier inn?

The living situation is untenable. While the 14 year old is going through a change, its unacceptable to be violent towards his younger siblings. This ineeds to be dealt with first. He cannot control his outbursts and it could also be assertion of dominance in some way. This is really not a safe situation for your youngest child or any of the younger children. He will become more violent. The siblings are already walking on eggshells. You will be next, his father needs to tell him it stops or he's out . If any of his brothers say something at school then you'll have social services involved and you'll be looked at as not prioritising safety of your 17 month old.

Get your other half to move out and take his kids and keep your baby safe .

This is a good idea

If the living room was suitable for the boys to sleep in, then its suitable for you two to sleep in until you do manage to live somewhere bigger.

Unfortunately op, it is the adults who have created this disaster, (mainly your oh for having 4 children he clearly cannot afford) and it is thus the adults who should take the brunt of the problem. The living room for you 3 sounds like the best option, freeing up a bedroom for the 14yo. Yes, he’s being a pain, but he is understandably very very frustrated especially with whatever happened to his mum.

Must be, where were all 3 older boys sleeping when visiting dad before he met op?
he downsized when he met her and had a new child?

You can't.
I would happily do it as a daytime resort.

In summer (sometime in winter too) our oldest kids lived and slept in a tent. Day and night.
Sadly we don't have a garden anymore but they were very happy teenagers. They have left home with fond memories of tenting.

You can’t put a 14 ya old in a shed.

Yes, he most properly can't use a key being 14 yo.
Some daytime privacy wouldn't harm, would it?

This is the sort of idea.

While you’re waiting to move, is there any way the boys’ room could be partitioned to give the 14 year old some privacy?

And what about at night? He has nowhere to sleep. He would be locked out if he was in a shed and I for one wouldnt be on for that.

A 2 bed house was never sufficient in the first place

Walk?

A shed could be used in daytime simply to get some room and some peace.Homework must be hard to do with 3children in one room and another child around.

Why isn't the living room an option?

Why did your OH only consider a 2 bed in the first place? Even before you had a child with him, he would have already had 3 children so most people would have looked at 3 beds minimum.

I am not sure you have a good catch here if he was so happy to start a new family and only move into a house that supports one new offspring rather than the existing 3.

Ha, my DH’s brother did his whole teenage years living in their log cabin in garden! He still talks about it fondly now. IME most teenagers don’t need to routinely use the loo overnight but he could just come inside if he did 🤷‍♀️

Well, the other option is you split - he houses his 3 older boys, you house yourself & the youngest.

Who is on the mortgage/tenancy?

So your plan is? It sounds very unsustainable.

You can’t put a 14 year old in a shed. How wood he get to the loo apart from anything else.

The father needs to actively parent in this situation.

first, he needs to address the violence. That should 100% not be tolerated. Pure punishment may or may not be the best approach in a high stress situation. The behavior might best be addressed with a mix of natural consequences and empathy.

second, he needs to stop the younger children from messing with the older teens belongings. They absolutely should not be entering the tiny slice of space he calls his own. His things belong to him and should be off limits. Being upset about children getting in his way is not unreasonable. The only problem is the way he is responding.

put tape down on the floor if you have to, but a 14 year old needs his own area.

Out of left field suggestion but could you get a little garden pod for the 14yo? A Hully Pod or something? Then he could have his own space.

Why not? Either for you or even the eldest?

Why is it not an option? Are the step children going to be with you permanently?

Your husband needs to sort out a more suitable living situation for his 4 children. You’re talking about a teenager who’s probably been through some trauma to be suddenly living with an extremely part time parent. He will be under significant pressure at this point in school.

of course he can’t continue to be violent, but he, and everyone else needs some space to be able to decompress, it sounds horrendously claustrophobic.