To want SD’s room to be multifunctional?

Not only does OP pay for the house but she’s now providing the bulk of the CMS payments her DH can no longer manage. Which is the reason for downsizing. If she’s the breadwinner and has not only her own family, but her DH’s ex relying on her income then l think that’s absolutely enough reason for the decision to be hers.

Nothing to do with CMS. Yes she could. For the disabled kids. and if she worked a few hours a week she could also claim somethings else that I can't remember.

Well that’s not comparable is it? Because in your example the child would be the OP’s biological child rather than SC.

Obviously use the room. SD should understand that being part of a family is pitching in. It would be so unfamilial not to let you use the room while she's not there.

I’d give her the choice of a single bed in the small room or the bigger bed in her room which will be completely her room except you will use and then leave an empty desk when she’s there.

If she chooses the small room, then you can rearrange ds’s bigger room so you have a completely tidy corner partitioned off with ikea kallax units or something. You can make it private and then you don’t have to worry about the tidiness of the rest of his room.

Yes, I don't think there actually are many threads where one breadwinner is functioning as essential financial security for two households who are between them weathering an appalling mixture of cancer, multiple DC with AN, problem debt and school difficulties. It's a pretty niche set of circumstances. I will of course take this back if anyone can show it comes up all the time.

With how much your SD has been through recently I think you should work in your room or in the kitchen for now and review the set up again once everything has settled. I think you need to prioritise her feeling secure for the moment.

She was trying to emphasise that nothing of the girl's is really hers. Her father and the OP own it and merely let her use it (and could stop her at any time). It's more territory marking and letting SD know her place is way behind the OP.

Can you have the largest room and put a desk in your bedroom? DS the second largest room as he lives there full time, and SD have the box room as she is only there occasionally. It's unfair to put DS in the smallest room when there's a larger room being unused most of the month. It's also unfair to expect to put the desk in their personal space when you can put it in your own personal space.

CMS income is not counted for benefits, so SM wouldn’t necessarily be entitled to any other benefits if CMS stopped.

Equally, if OP were a man wanting to drop maintenance by 6/7ths to his struggling XW who has their DD the large majority of the time in order to ensure they could maintain the same lifestyle, there would be plenty said about that too!

Yes, I agree. In light of this additional information, I've changed my mind on the situation. That poor girl.

What other threads?

Yes, and you're supporting yourself, DH and his ex, your DS, SD and the ex's unrelated children.
You're going to have to focus on work.

And her DP is 'dictating' it's my daughter, she gets to decide whether you use an empty room.

Which one is reasonable?

That OP gets to work in a suitable space or that the room sits empty?

Why is DS getting the box room when he lives there full time? It’s his only home.
DS gets middle sized room. DSD gets box room, which you can also use to work at…. can you get a high sleeper with a desk under? Not unreasonable for the room to be multifunctional if she’s only there every other weekend. You’re not even using it to work when she’s there.

@SamphireSupper sorry just caught up. So SM has 2 more children who are disabled. From a man who's disappeared and left her in debt. She is surely 'entitled to ' benefits that would negate most of the 700 your DH is providing to pay for 2 kids that aren't his. His daughter is clearly suffering (from what you've said about school etc) - the last thing she needs is a shared room with her SM. Its not just a 'desk in a room' - its her personal private safe space in 'her dads house'.

And op that’s not a dig at you. I really feel for you and the whole situation must be horrible.

its the posters suddenly stating paying means you have the ability to dictate, when they are oddly not saying that on other threads…

Sorry, just to clarify what I said here in case it was unclear ^^ I didn't mean that you should just find any old place in the house, OP. My point was meant to be that people can be flexible so your SD shouldn't mind you sitting at 'her' study desk on days that she's not there - in fact she wouldn't even know! You could assure her that you wouldn't be touching any of her stuff that she leaves there.

It's very likely, because it was me, and I can say that unequivocally. Disbelieve it all you like.
However.
More information has been provided, and the SD is clearly having a very difficult time and her school attendance has been hit. So I'm inclined to think that she needs an extra level of care and support. I would therefore suggest whatever the OP and the Dad can do, the better

Ok, you have a house with an empty room. You WFH but have no study or private space to use.

Do you ignore the empty room and sit in the kitchen with everything going on around you or do you make use of the quieter, more private empty room away from the hub of the house?

It makes no difference if you are male or female. Which space are you choosing?