To want SD’s room to be multifunctional?

I think you need to be creative with the desk get one that folds flat against the wall and have a set of drawers on wheel where you wheel out into your room along with the chair
so when she’s there at weekends it’s just a normal room
something like this

You may be surprised at how much the ex is getting through benefits - potentially £500 per month for each child if they qualify for DLA, and extra money from universal credit if caring responsibilities affect her ability to work.

You paying the £700 means that is money she cant claim in benefits.. unless she's not declaring it. ?
But with two disabled children she should be eligible to claim a fair amount, so you giving her £700 instead of £ 100 is you pissing away £600 and as it turns out, your home !!!!

Ok, but why are you supporting the ex and her 2 other children?

She doesn't get the option to try to tell you no. It makes perfect sense.

Op...your dp
Does NOT have you and your sons best Interests at heart
I'm sorry but he doesn't
He should not be allowing you to pay that £700
That is his responsibility..
He is putting himself and his dd before you and your son .
Please tell us your not married to him ,and he's not on the deeds of your house

I think you're going to need to take some professional advice about your finances, OP.

They're taking you for an absolute ride, and you're being way too soft, OP. Your and your son's lives are in upheaval because of some unrelated child and a demanding partner who cannot work properly (yes, I know he's ill).

You're pretty much single-handedly supporting his ex, that's ridiculous. She's not your problem at all. Why are you doing this? Spend the money on your son and yourself. You're downsizing your house because of them; that's madness. In the new house, your own son, who's there full-time, gets a box room, and an SD, who's there two weekends a month, gets a bigger one. And still this madam and her dad are unhappy, because you want to put a single DESK there, which she can also use. Meanwhile, you're paying for everything.

This is crazy, they're taking the piss massively. I'd do this: stop paying maintenance, he can pay the 100, not my problem, not my business. Then I either wouldn't downsize, or if I would, I'd take the box room for my office, a larger room for my son, and SD can sleep on a sofa twice per month. If she and/or her dad don't like it, they'd be welcome to leave.

That's really sad ,I'm sorry to hear that .
But I feel you are making the wrong decision here .
His cancer will massively impact your son ,and then your son has to move to a smaller home as well as a smaller bedroom.
No ..
He needs to pay the mum the £100 ,as the calculator says
And you op ,need to focus on your child
You are not responsible for his ex wife's horde of children..Sen or not
They are not your responsibility, they have two parents who are perfectly capable of working things out together.

This.

You need to start protecting yourself and your son.

So you are now supporting two adults who work minimally and four children, three of whom aren’t yours to the disadvantage of your actually child? And you’re not even married? I think you’re either a saint or completely mad. I’m afraid I wouldn’t be doing what you’re doing.

So did he move in to your house that you owned

This is mumsnet, of course stepchildren come first. That’s evident by the number of posters who think it’s fine to leave DSD’s room standing unused for most of the month, while OP works from her DS’s room, rendering it unusable for him while she’s doing so. It’s batshit.

He's done a number on you ,op ,to make you think this is your responsibility

What issue does dp have that he can't work
Did he move in to your house ....then became ill and couldn't work ???

So...the Dad of the younger children is surely responsible, not you?
Why are you supporting these children?

I’m the first to say that NRPs shouldn’t prioritise cutting maintenance above all else.

But keeping paying £700 at the cost of losing your joint home is madness.

£100 is very low but there must be a happy medium? What could he pay and you guys keep your home, which works for everyone? Could different cuts be made in lifestyle to make this balance?

Otherwise you (the OP) are trading in your security to support another woman which is bonkers.

So the £600 is coming out of your pocket op
As he only needs to pay the mum £100.
Would universal credit not top the mum up .
If £100 is what the calculator says he has to pay ,then that's what he pays .
What happens when she's 18 ..will you continue to pay the extra £600...
What if mum is relying on it for years to come ..what you split up with your DP ..the mum wouldn't get her extra £600 then ,and she would have to manage.
Why on earth would you let that effect the life of your own son ,so he has a smaller house and smaller bedroom,so the mum can stay in the same situation.
And that's assuming he is actually paying the mum £100 ...or op ,or you paying the full £700 yourself???
Your basically supporting 2 families single handedly.
How long can you keep that up for ..

This. I have three children in a four bedroom house. All are mine and DH’s. We also have family to stay around once a month. One of the rooms is massive (but can’t be split) and the others are all small doubles. The child that has the biggest room has to move out for guests. That was the deal, everyone is ok with it. In a normal family home it’s not always possible for everyone to have exclusive use of their own dedicated space, there has to be some give and take.

Can we have a wave of people expressing how unfair this all is for DS now, having this upheaval in his life so that DSD can have the same standard of living at her mums house?
or is it just the step children we have to show concern for?