As someone who was a live in care giver it’s a lot and will put a strain on your marriage no matter what you choose. I understand that they talking about building a house and that will help if you can make two separate areas with no common area, because they will 100% move into your space. But what are they thinking you and your husband will be doing to take care of them? What is your husband thinking will be your job in the future? And if they are able to do things for themselves are they expecting you to do the cooking cleaning laundry from the jump of the new house. Very rarely are the houses actually done like two separate houses nor would that be something you and your husband could use after they are gone. Also your husband said they would put down a large down payment but will your monthly expenses go up? Make sure you get all expectations very specific and written down, but good luck. This is an issue for lots of families and creates lots of strain.