Well, the living situation of the parents-in-law is none of our business. Whether a bungalow can be built or not, neither. ...
However, an honest and ruthless opinion and answer are requested here, also whether the facts are realistic.
I often read here that the money from well-off parents, who still have to sell their stately home, is included in the financing. Then a small basement granny flat is shown in the floor plan draft, which, however, does not include anything of a retirement home. When asked: for the parents/parents-in-law, when asked whether they already know of their "luck," no answer comes anymore.
In this respect, I do not see it as realistic: why should well-to-do under 80s live worse than any of their tenants? Ok, I have no idea about their rental situations or infirmities, but when I look at my many neighbors here, who, over 60, have put up a smart 120 sqm bungalow, so many others over 70 still know how to live and spend their own earned money alone, these thoughts of the original poster make me shudder! It would never occur to me to count on 50 sqm for my parents. A caregiver might possibly need a room, accessibility also consumes some extra square meters.
The topic of "inheritance" is closed in one sentence: let the seniors keep their money for their care and their long life.
In this respect, I do _NOT_ see the project as realistic!
Edit: the income is still great and a construction project is feasible!