Multigenerational house (two parties) on existing plot

Everything is all well and good, but building living space half-finished that might maybe be needed in 15 years is just a stupid idea. That needs to be solved differently.

Since inflation is rising exponentially, things could look completely different in 15 years. :p

Six hundred fifty times zero four and eaves height three five: as charming as I find maisonettes, I can hardly imagine a constellation in which I would consider the latter for a two-story building. Your idea of a house far from development plans with a roof shape conforming to the development plan reminds me of my Fiat Uno, which had the same turn signal lever as the Ferrari F40. Development plans are overlap-free. As popular as the desire to be allowed to build exactly like you can see within spitting distance is, it regularly serves not one bit as an "argument" for an exemption.