To want SD’s room to be multifunctional?

Sorry, I cross posted with my guess about what was happening.
So the SD sounds very vulnerable. What an awful situation, poor girl.

OP says that the ex is supporting SD to be back at school full time. This would indicate school refusal, or some sort of illness and/or anxiety..

Have I missed why DSD is not at school?

That’s what I mean by “losing your home” - moving to a different home that doesn’t quite meet the family’s needs. And it clearly doesn’t.

And you’re losing security by moving to a smaller home in these circumstances as it won’t be worth as much.

I would be rethinking “maintaining our lifestyle” as a priority as well as the large amounts of maintenance.

13 is a tricky age. Her dad is ill, he's moving house, he's got a toddler who is claiming his attention... now she's not even allowed a fully private space in the new house. I see your point OP but I can also see her POV.

Is there really no other house you can afford that allows her to have her own room while giving you a decent home-working space?

Best wishes to your DH for his health.

I don’t have a spare 10k to build a garden room, and I’m not spending 50hrs a week in a cupboard under the stairs when there’s a perfectly good room sat empty!

Quite. Yet another thread where common sense goes out the window because there is a step child involved. You can't keep a bedroom/shrine unoccupied the majority of the time just because a step child uses it occasionally. No wonder this country has a housing crisis.

ETA: sharing is a way of life for many families

Ok, so in this turbulence, your SD has not been at school?
This really puts a different complexion on things, about how vulnerable she is.

Exactly this.

Circumstances are difficult, so everyone in the house is unfortunately going to have to belt tighten, and not be able to have all the things they might like. DSD as a member of the household can no more be immune from this than DS can. Shit happens, has happened.

DPs unfortunate illness makes it even more important that you as breadwinner have a suitable remote working setup, and DSDs bedroom is the least worst option. She can always have lockable draws or storage if she has particular worries about privacy.

So definitely can’t be done at the kitchen table!

How do you know that people would be "up in arms" if the sexes were reversed? That's quite an assumption.
My advice would be the same.

I think your proposal to work in your stepdaughter's bedroom is the best option, in these circumstances.

Is her concern that you will be in her bedroom (forgetting that you already go in there to clean)? That she won't be able to decorate to her taste or leave stuff lying around, if she currently does? Rather than a desk, which you share, could you plan to get some sort of cupboard for all your work stuff so it would be shut away completely every weekend?

@Geneticsbunny why on earth should she? She's funding absolutely everything (including 2 kids that aren't even her DPs let alone hers) and she should spend 18 hours a day in one room for that? No chance, how on earth is that fair?

I think this is perfectly reasonable, you will only be using the room when she isn't there and you can clear your work things out. I am sure she would prefer that to the box room and a single bed.

I don't know what "we" want? Who is the "we" in this instance?

Cant you put a desk in your bedroom?

YANBU. Absolutely not unreasonable AT ALL.

The only way is which you ARE being unreasonable is to continue paying £700 towards funding children that are not only not yours, but not even your DPs! SEN or not, they are not your responsibility. Or his for that matter.

There are 2 options, either SD has the bigger room and sucks up sharing a desk (that she's not even there to use) or she has the box room and shuts up about the bloody double bed.

As for your DP, whilst the fact he is ill is difficult and upsetting, he HAS to understand that YOU alone are funding his lifestyle, and your job has to be a priority because it's literally keeping a roof over his head and food on the table.

They both sound incredibly selfish

I wouldnt even be discussing this beyond a token "i see what youre both saying. But no, desk is going in SD room"

can't do right for doing wrong. somehow, sharing a room with SD is horrendous, but maintaining support for SD mum, which ensures SD other home is stable, is madness. which is it?! we want the best for SD (at the expense of DS) or we don't?!

OP, I think you're a saint and your plan sounds like the best possible outcome for everyone involved at the moment. What a tough situation you've found yourself in. I hope it all eases up soon.

If it was a bloke who came on here and said their wife had become ill and as he was now the breadwinner he was going to unilaterally decide his SC's bedroom doubled up as his office people would be up in arms!

There have been a lot of suggestions as to how else she might be able to work in peace.

And also a lot of suggestions as to how she could discuss this with her DP and SD to protect the relationship with the SD so that it's compromising and finding a solution as a family rather than her stomping all over what they want.