What horrible mothers you previous posters are. OP, you and your husband sound wonderfully supportive. 23 and pregnant with a man she is not living with, it’s not exactly planned, is it? You are giving all you can to help a new baby and a new mother have a stable home. It is very weirdly British I think that everyone has this idea that intergenerational living cannot work unless you live in a castle where everyone has their own wing.
Me and my husband lived with my parents for 9 months after my daughter was born. (Covid paused work on our renovations on our own home). We had one bedroom for the three of us and we shared a bathroom with my younger sister. My parents had an en suite so that was very helpful.
We did things like set up a TV and a kettle in our bedroom so we could have some space, and avoid crowding my parents in the living room. We agreed on a routine for kitchen use and shopping/cooking. We all did our own thing for breakfast and lunch and then we had a rota of who was cooking dinner, and we ate together as a family. We paid a weekly contribution for bills. Most of our belongings were in storage during that time to avoid crowding their home with our stuff.
She was my baby and I was always the primary carer, but it was absolutely brilliant for her to have a close bond with grandparents too. They would take her for a walk or for a cuddle or even offer to let me nap or me and husband to go for a walk on our own, ALWAYS AT A TIME THAT SUITED THEM. You are in control of how much you offer to do in terms of baby care.
In 2020 all the baby groups were cancelled of course and we were home all the time, but we made it work by using our bedroom as our own living room too. From this point of view it makes sense for your stepdaughter and partner to have whichever room is bigger, as that will mean more privacy for all of you.
You don’t know how the birth will go and how she will cope afterwards, but you can ask her to take the initiative and make sure she agrees to check out playgroups, breastfeeding support meets, swimming lessons, baby massage, postnatal exercise programs… whatever gets her out of the house and part of the community of new parents in your area.
If you only have one bathroom the adults should try to agree a schedule for access in the mornings and evenings, and everyone will have to leave the place clean when they finish so nobody gets annoyed about showers and toilets. Of course stepdaughter will need some help or flexibility with this in the first 6 weeks after giving birth. They should have two washing baskets in their room, one for baby and one for adults, and they should always be in charge of their own laundry. Always in their room never lying around the house. A heated airer for their room too to help keep on top of laundry if the weather is bad. They need to buy their own detergents.
I can’t think of anything else just now, but I think you are wonderful parents and I wish you all the best.