You always say that she has to move out at some point. But why exactly? I don’t understand.
As I understand it, she has to move out because she has then used up her capital, has no ongoing income to cover living expenses, and therefore the bungalow has to be converted into money.
Calculate how much the bungalow may cost at most, assuming she will live to be 95 and needs the amount X monthly in addition to her pension – alternatively with nursing care service from about 85. Then the forum members can give you an assessment of whether/what is feasible.
I would not consider it a given that your mother necessarily has to move out at some point (unless foreseeable due to an existing illness). There are plenty of elderly people who manage on their own into old age or use outpatient nursing services.
The preservation of value for the grandchild would then be the bungalow and the property.
In addition, you could make a list of her living wishes without her specifying the square meters. Then you look for some nice floor plans that meet all wishes in a small space and show her that living comfort does not necessarily have to have much space.
As I understand it, she has to move out because she will have used up her capital by then, has no ongoing income to cover living expenses, and therefore the bungalow must be converted into money.
With a paid-off property, this is practically not possible. Even basic social security is possible with one's own property. The lifestyle is probably not appropriate to the income. So second problem!
Actually, I am expecting the thread "New Bungalow Construction in Old Age - and then? Or Multi-family House with my Son" in this forum. Since you take care, the question is: does she take care as well or does she just let the son do it? It would not be a new situation of wanting to have, but not taking care of it, instead passing on the task. This does not help you further, but maybe simply confront your mother with this fact that she is making you co-aging.
I would still distance myself from the multi-family house idea. Completely out of focus is the question of whether the planned five apartments will even be approved. With a maximum of three, which seem "certain," the whole construct collapses (two rental apartments each, creating income for you and mom).
I almost have the feeling of a fundamental error in thinking with the so-called house-building forum "running gag" of spontaneously developing a multi-family house usually on an existing plot: namely, that on the slab already existing for the ground floor, the upper floors would basically create bonus apartments that would only cost a few rows of bricks to build but would then be fully effective in terms of rent repayments. In other words: you take a two-family house in your naïve construction calculation but a four-family house in the loan servicing calculation, and suddenly everything pays off as if by magic, and if the nasty zoning plans were not so humorless, then actually everyone could just build a high-rise and enjoy a relaxed retirement with roasted pigeons — if it weren’t for the word "if" (in the form of such spoilsports like me, who utterly baselessly claimed that there was a flaw in the reasoning).