When dh and I were first seeing each other and not yet living together, I didn't have a room for his kids. When dh first started sleeping over, we set up a sleeping area for the kids upstairs but they had to bring sleeping bags. That was just how it was with them anyway since dh was living at his mothers at the time and she didn't have room for dh and 2 kids...
Once dh did move in FT, I set up one bedroom with bunkbeds for the kids, then 5 & 7. We also moved our bedroom upstairs and left the 3rd bedroom set up for the cats
I had 3 cats at the time and preferred to have their stuff all in that one room.
When DSD and DSS hit 7 & 9, we thought they might appreciate separate bedrooms. We found another set of bunkbeds at a garage sale with matching dressers and end tables and snatched it up. I moved the cats' stuff into my laundry area. Right around then, I got pregnant
We sort of figured the kids would love having their own rooms but that was not the case. They preferred to be in the same room still... We almost made DSS's room DS's room... but figured we'd have ds in our room anyway so we just left it...
Now that the kids are 10 & 12 they do like their own space. They do know full well that they will be sharing their rooms with their younger siblings when the time comes
FWIW, the ex's fiance didn't make room for the kids until just recently, like the past 3 months. He has a 2 bedroom, and his 20 YO was living with him until very recently. The fiance has finally started an addition for another bedroom and got his 20 YO to move out. So the kids slept on his nasty living room floor for at least 5 years... However, most of the time the ex lived with her parents so at least at their grandparents they had beds.