That box, at the head of the bathtub was where the Water Heater usually went.
Not in the middle of the house.

And, where you have physical items that need to go between walls, you need to set the measurement around them.

It is not 5'-6" between the right faces of the wall next to the bathtub.
It is 5'-1" between faces of the studs next to the bathtub.

If you are going to use trusses, then this floor plan is pretty good, if you are going to stick frame the roof, you want to have a continuous wall under the ridge. Which you almost have. Oh, we could definitely frame this, but the attack wouldn't look as neat.

And, about affordable housing:
It really isn't about building a cheaper/less expensive house, it is how much the land costs. Most places have very expensive land, with a very cheap house built on it. Even if it is a large house.

(shoddy construction eveywhere)