Step parenting challenges

[Loz2470]

I’ve been with my partner now for just over 18 months and he moved in with me last summer. He has two daughters (9 & 6) who we have two nights mid week and then every other weekend (the same time they have with their mum as they stay at their grandparents one night a week too). This is a new arrangement from when he moved in as previously he used to have them only at weekends (ad hoc) and never during the week.
They share a bedroom, and it’s decorated in the exact way they wanted it. I am currently pregnant with our baby and the nursery will be the small room but I’m starting to feel like the baby should have the larger room as she will be living there full time and will need the space. I know this is something that my OH feels strongly against (as his ex has done this at hers with her son).
Another thing I’m struggling with is the mess. Before they moved in it had always just been me and the house was immaculate. I knew when they moved in this wouldn’t be the case anymore, but I massively underestimated by how much (he also knew how much of a tidy person I was before he moved in and how mess doesn’t sit well with me). He is really messy and so are the kids and I feel like I’m at the end of my tether! I don’t think it’s unreasonable to ask them to keep their room tidy (toys away and dirty washing in the laundry basket) but he turned round at the weekend and said I should go easier on them as their mother does everything for them at hers and he doesn’t want them to not want to come to us because I ask them to do stuff.
I feel like I’m at breaking point at the moment and with the baby coming along, I’m not sure how much longer I can maintain this. I’m just after a bit of advice really?

You say he moved into your house? Then yes I'd be giving my child the biggest room.

Im in a similar situation. My partner doesn’t live with me but does bring his two kids to stay at mine one weekend a month and a bit more during school hols. He/they live an hour’s drive from me. I like the kids a lot but my house is too small for all of us and they are incredibly messy and will do stuff like play football in my kitchen unless very closely managed (I have one shy quiet child who mostly reads, so big contrast). We’ve had lots of, um, energetic discussions about this. I can’t afford to move in my area and won’t move somewhere not nice/not convenient to accommodate someone else’s kids. Especially in current housing market. Our solution has been that my partner - who is also messy - works on this in himself and with the kids. I can’t be the shrew nagging them to pick up dirty pants or take their plates. He also does all DIY related to his kids’ needs and pays for at least half of the DIY stuff. He’s made them a bedroom in my basement. He built a cabin bed for them and shelving and put pictures on the walls and added space saving storage for toys. Note he doesn’t already live here and doesn’t plan to so I always have the leverage of ‘well if you do want to keep bringing them.’ I also gave him the choice of putting in £60k so I can move into a larger house in the area. He chose DIY. Keeping everything the same was never a choice. He wasn’t surprised when I started pulling him up on the mess as no one else invites him and his kids to stay.