Not crazy! You can look at the spec sheets of the products you're considering to answer your questions. The Kohler underscore can be 19 high or 22 high -- it's the same inside but one model with drain set below the floor and one above.

We added a grab bar to help with stepping over the high side.

You are gutting a bath, spending $$$$ and the first question? Show the bath you are gutting. It's a 5 x 8 classic in an older home? In a mid century ranch?

Second......Is this the ONLY tub in your home? Who uses a tub , besides your son.

Third......."a 20 something son " with plans to remain in your home for how long, or is in some way compromised and requires care?

Do you have guests that use this bath? Do they want to climb over a tub edge?

A tub doesn't "get lifted up" it sits on the floor you have, just as the tub you have sits on the floor. You have a right or left drain, yes? Soaking depth and water level, can have little or nothing to do with the height of tub sides.

Kohler Underscore is a nice tub, about the deepest......but you need to address the entire bath, along with whatever tub. Along with consideration the stay of the son who shall USE it

Show the bath , and best? Draw and measure every last wall, window, entry etc, and show that as a jpeg, uploaded in the comments below.

Kohler Underscore specs

All product dimensions are nominal.

Notes

Installation:
Drain location:
Basin area, bottom: Basin area, top: Weight:
Minimum floor load: Water depth:
Water capacity: Minimum flat for door:

3-Wall Alcove
Left
46 1/4 x 22 (1175 mm x 559 mm) 52" x 24" (1321 mm x 610 mm) 105 lbs (47.6 kg)
56 lbs/ft2 (273.4 kg/m2)
15-1/2" (394 mm)
68 gal (257.4 L)
2-3/4" (70 mm

Your son , from what I can read will have to deal with a 60" long tub and his knees will have to bend . How long is heplanning to live there ? I also wonder if he has some issues. I love my tub adn I have to bend my knees to soak too . Go look at the tubs with ameasuring tape to see but we need a lot more info to help with the actual tub.. The tub height stays the same inside no matter what tile you put around it. I think from what I read you need to aslo hire someone for this job.The more slope in the back the longer the tub needs to be to get the bathing well he wants I have a nice slope and 46.5" of well in my 60" tub my knees bend I am 5'9" tall . I do not think the weight of the acrylic tub will factor in much sonce you have a cast iron one now So look at tubs with little slope to fit in thta spot .

I am 5 ft 2 and have a 66" tub 22" tall (outside measurements) that is perfect me to have a good soak. When looking at tubs I looked at the outside measurements and what I would really get on the inside up to the overflow. For me it is always been the taller the better.

Jan, to answer your questions...

1) Not my 1st question about remodeling this bathroom -- I had to change usernames when I bought a new computer; my initial questions are under a different name. Nearly all decisions have been made on this remodel, except for the bathtub and what color to paint the wall behind the towel bar.

2) The tub/shower combo in the hall bath is not the only tub in the house -- but it is the only working shower. I have two other showers (installed in remodels 10 years ago) and have continuing issues with both. One has a slow leak and the other gives only cold water.

We are still waiting for the to scale plan showing every window doorways where those lead and every meausrement clerly marked on graph paper if that is easier .Posted here in jpeg format in a comment DO NOT start another post . Until we have that we can't help. BTW IMO get o work on those other bathrooms now before the whole place gets flooded

Floorplan attached. I had the adjoining bedroom and hall bath in separate files -- not sure how to merge them -- so I redrew the hall bath onto the corner bedroom drawing. The dimensions of the hall bath are 96"x91"

Jan,

To answer your question about who uses this bathroom. Over time, various guests, family members, and a live-in nanny have used this hall bath. The tub/shower, with a 14-inch high step-over, was perfect for my son when he was 2 years old, but a challenge for my parents when they were in their 80's.

Is there a "perfect" step-over height for everyone? I doubt it. I'm guessing it is somewhere between 14 inches and 22 inches, based on what manufacturers are selling.

I would like to think that if I sell my house in two years, the new owners won't rip out the new tub and new vanity, but who knows?

Adding a floorplan of the existing bathroom below. The toilet space is 33-1/2 inches wide (meets code on both sides). Cannot lengthen the five-foot long bathing space. Sacrificing space from the adjoining secondary bedroom is a non-starter.

one more thought -- how big is your water heater?

I switched to a tankless water heater 10 years ago.

The tubs on my "short list" range in volume from 44 to 60 gallons of water.

The smallest is the Kohler Bellwether 60x30 cast-iron tub with a 15-inch step-over ht and 44-gal volume.

The largest is the Americh Cloe 6032 acrylic, sold through Pacific Sales, with a 20-inch step-over ht and 60-gal volume (similar to the Americh Kent)

I have eliminated the Bellwether 60x32 because the drain is set back 12 inches -- it requires a tub spout longer than 7 inches and would be more work for moving the tub drain location

I have not yet tried the Kohler Irvine acrylic (sold through Fergusons), but will likely eliminate it because the step-over ht is 21-1/4 inches. I have already eliminated the Hydro Systems Sydney 6032 with a step-over ht of 22 inches.

I am thinking about the Hydro Systems Sydney-17 acrylic.

Check out Kohler’s Archer Tub. Its available in 60x30 and 60x32 The water depth is just over 15”. My daughter had one in her hall bath and loves it.

Below are some of its features listed on Kohler’s website

• Comfort Depth® design offers a convenient 19" step-over

height while retaining the same depth as a standard 21" bath

• Integral lumbar support

• Slotted overflow allows for deep soaking

• Integral apron

• Integral flange helps prevent water from seeping behind wall

and simplifies alcove installation

• Textured bottom surface

• Coordinates with other products in the Archer collection

Kohler has 2 different model numbers for their Bellwether tub. We installed Model K-837 a couple years ago and the drain is not set back like Model 875.

I was so glad I saw that set back drain spec before I ordered because it would not have worked in my bathroom because my drain piping could not be moved. The water depth in the Bellwether is only 9”

The first two tubs we tried were the Kohler Archer and Underscore, which are displayed side-by-side in our local Kohler Showroom. My son did not like the armrests on the sides of the Archer -- he complained that it makes the tub feel narrow. But maybe it is worth a 2nd visit and retry.

I needed a hand to help me enter/exit both models, maybe because they are displayed with sliding glass doors. (I'm planning for a shower curtain.)

After two months of researching bathtubs, we decided to trash the tub and circle back to the plan for a standalone shower (3' x 5'), with one change. We are adding a built-in bench to double as a partition between the shower and toilet. We have a built-in bench in the downstairs guest shower.

As part of the tub research, we spent 3 weekends going to open houses of new construction homes (ranging in price from $2M to $6M, with 4 to 6 bathrooms each), within a 1/2-mile radius of our home. All but one of the homes had at least one secondary bathroom with a 5' alcove tub/shower. Most of the tub/showers were fitted with a shallow Kohler cast iron tub and sliding glass doors.