To ask 23 year old to give his bedroom to step son?

This. Until DH sorts it and either buys you a bigger house or chucks him out

The thing is if he couldn't afford the flat without his gf then he won't be able to now either. He can either present as homeless to the council and be placed in temporary housing and be at the top of the housing register for affordable housing or get a house share but your partner won't be very happy at all which I can understand. I can understand your point to though tbh. We faced a similar situation tbh but a relative stepped up and helped out in the end

So his mum had full custody of him in a 2 bed house and then had a daughter with the expectation she would have his room when he moved out?
If she is 7 he was 16 when this plan was put in place and as soon as he moved in with his gf his room was given to the next in line.
You also have no room as you haven't had to accommodate him due to him living at mums?
Why is this all on you now then what's his mum doing to accommodate both her dc? bearing in mind that's been his bedroom for his entire childhood perhaps his mum and her daughter should share a room in their 2 bed house and he share with his mums partner as she seems to be the one that's has caused him to be on the verge of homelessness and now passed that problem onto you.
I wouldn't want to sleep in a separate room to my husband and clearly nor does his mum so I don't see why it's down to you to do the sacrificing. Surely his mum also has a lounge or dining room too.

Do you own your own home? Realistically either you all move house with your DSS in tow or he needs to find appropriate accommodation.

Why would anyone move house to accommodate a temporary adult who has asked if he can stay for while, he wasn't invited to move in permanently it said he came to stay for a while after a break up.
Up sticks and moving house is a tad extreme given that this wasn't planned and op has pretty much had this put on her out of the blue.
Suppose they move house and he moves out a month later?

THIS. It's awful to be a girl going through (and beyond) puberty and having to share with a brother. I had this experience until I was 16 (shared with 3 years older brother until I was 9, then he had to sleep on sofa bed downstairs and I had 9 years younger brother in with me) and we finally got a house with an extra bedroom for me to have my own.

My younger brother has two daughters (now 23 and 18) and I've talked to him about how awful having no privacy was - he totally gets it because of having his own girls.

Oh god here we go

This is a reality for many families. He will have to game and watch inappropriate films somewhere else. As a parent you will wait to do these activities when your kids have gone to bed. He lives in a house with young children and will need to accommodate that.

Is a garden bedroom an option? Like an office pod?

Do you own your own home? Realistically either you all move house with your DSS in tow or he needs to find appropriate accommodation.

got to be a compromise-your dh an dss have to realise a girl of her age neesd privacy

With rental costs these days can dss even afford to move out?

Ask him if he wants to share with his dad or his younger sibling (on the understanding that as he’s sharing with a child, their needs will always trump his). You will likely find him on spare room before long, looking for a room to rent.

THIS!!!

I’m sensitive to the anti-SC narrative too, but I think in this case there are three siblings living together and the girl needs to be given her own space. I wouldn’t have wanted to get to grips with periods and bras while sharing a room with my brother (or sister tbh). This means the boys need to share. Mum/SM sounds like she is considering all the children and DH to make the family unit work ❤️

Why?

I’m actually not keen on the DS and DSS share idea - it’s maybe better than what you’ve got but I wouldn’t want my 10 yo son sharing with an adult male, even if they were his brother (but a brother who he hasn’t grown up with).

Absolutely Dd needs her own room. She shouldn’t be sharing with her brother.

Some options seem to be -

1. the living room becomes a bedroom for DSS, or he has a sofa bed in there that can convert it from a living room to a bedroom at night.

2.You and your dh take the living room, giving one of the kids your room.

3.DH shares with his eldest and you share with Dd, giving Ds his own room.

The answer definitely isn’t for a 12 yo girl to be sharing with her 10 yo brother

This. DSS will probably not like sharing with a 10 year old and will probably look to move out. Point out he is still welcome to come around some evenings and to share a meal etc.

It says something about our male-centred society that a pre-teen girl has to give up her privacy and dignity, but an adult male can't be asked to watch his preferred films and games in a slightly less comfortable place or timeframe for him.

DSS needs to not watch or play anything inappropriate in his room and share with ds, or you move in with dd and dh move in with ds. I’d offer those two options to Dh and see which he said. If he did head in sand we can’t do that, I’d move him out of our bedroom and dd in. She’s 12.

I have a 23 year old and no way would I be kicking him out. I'd find some sort of solution. Even if it was a caravan in the back yard. Is there anywhere you can curtain off a space for him? Cut a living room in half? It might not be the ideal and most comfortable solution, but it's a solution to everyone having their own space. If you really can't find a solution, then I'd be telling DH we need to move to a bigger home. Son working full time can pay rent to help fund it (but make sure you can carry it if he moves out and that rent disappears).

DS23 can not play inappropriate games or watch inappropriate movies if he doesn't have privacy for it.

Well that's marvellous news if the OP lives within half a mile of your house.

Not ideal but I think your only solution is:
Youngest son in smallest room
Mum and daughter share
Dad and oldest son share

Until someone/all of you are in a position to move or or convert another space.
If you have a floorplan to share someone might have an idea you haven't thought about.

True! I don’t disagree with that.

Your poor daughter. She needs her own space.

I agree but it’s equally the fault of the stepson’s mum who went on to have another child and now has no room at her house for her first child.

I would not have taken my children out of their room in the first place
He needs to move on, you've made him to comfortable, if you'd have made him sleep on the sofa, he'd have probably been gone by now