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)
I initially intended to have the WH there, but access is still an issue.
Note which side of the framing is dimensioned. The clear space between studs is 5'-1." I may actually increase this for a roll-in shower unit once I check some commercial options.
I thought about a solid ridge beam and rafters like the original 1947 house had, but trusses are the cheap and fast solution, and adding a stairwell for an attic and basement would add a lot of extra complications.
And yes, even I remember when land was as low as $1-3k/acre in my rural area. No more. Bare land costs more than my parents paid for 10 acres and a house almost 30 years ago.
The access to the WH is a door to the outside.
Insulated door in colder climates.
You want it there because water heaters burst all the time, and you want the water to flow outside.
The one piece tub/shower unit is just a hair under 5'-0".
So, the opening is that, plus 1/2" for the drywall on each side.
For most of the one piece units, you shim the unit to the wall studs where it needs support.
That million dollar house, is $100k house + $900k property. It really wouldn't make a difference if you could make the house for half price $50k. But, really $100k is a cheaply/poorly built house already.