I find the floor plan itself quite coherent, but I also noticed this huge dining area that just seems like "empty" space (between the kitchen and the dining table). If I interpreted the north arrow correctly, the bedroom at the top faces almost south, one child's room faces north(west), as does the living area. Normally, the orientation would be discussed here.
Just think about it: with a project costing €700k, is it worth having €400 rental income from a separate apartment as "support" for the financing? Frankly speaking, I wouldn't do it. I can only emphasize the point about garden access. I once lived in a two-family house, in the upper apartment. Especially when the children were still small and communication was limited, there were so many conflicts... In the garden, I have to add, there was a pond, so you could never leave the children unattended. So you were "downstairs," the older one was thirsty, forgot to bring something. So back upstairs with the little one in your arms, who screamed because she wanted to be outside... for no reason at all, back up the slope to the house, up the stairs, kitchen, drink, return. A lot of noise (literally!!) about nothing!
So, I would shrink the house a bit (by the empty square meters), I would scrap the separate apartment and live in the garden myself. See if you can get the bedroom facing more north, and the children in the south.