Might be moving a little fast
but I think the best plans for cohabitation are to build. What families do is put several dwellings on one plot. Several taller two story square footage space efficient dwellings can be built to replace any square footage hogging ranch style home if zoning allows.
I have scene some very well designed multi family properties, and not that I have experience blending families, but having more than one dwelling would allow for better separation of space. Things don't have to be going sour to enjoy quiet time off by yourself. In fact it makes more functional sense to me even if you aren't blending families.
How many mono marriages wouldn't have ended in divorce if there had been just a second dwelling for an activity space to for kids of for the adults of any household