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.