Hi
Really I did not understand the premises or ....
I insist, I can not find a relationship with the m2 you need to live comfortably, according to the order and the maximum volume allowed.
I am hard headed but for me that 150m2 / 400 m3 ratio is perfect for a house that supports a flat roof at 2.60 m (and almost too fair), now for a gable roof in snowy area, I really can't find the shape. (The regulatory slope is close to 30%). To be clear a room of 4 x4 m with a flat roof at 3 m has a volume of 36m3 but if I have to put a gable roof with 30% slope I have to add 12.80 m3 more or a third part is used In the attic.
In the case of 150 m2, the possible covered area to be used would be 100 m2
For my criteria almost impossible to achieve 4 bedrooms and others, in 100 m2 that is why my approach is based on something you said .... that you would seek to make the roof as low as possible to optimize the floor. I fear that this is not in accordance with municipal regulations.
However, I believe that today my job is to present a comfortable lay out adjusted to your requirements, then we will see how we are getting and where to get.