This ^^^ Or just build the little wall and stand it up in place...attach to ceiling and floor, apply vapor barrier and then cement board.

More than likely new construction won't make sense because of the current cost of construction, but if you are in a higher-end neighborhood the numbers may make sense.For the 2nd means of egress, what was the quote you got for the stairs and architect fees separated?   

I’ve invested with Fundrise, Streitwise, Groundfloor, Stairs by Groundfloor, Concreit, and Arrived.

I go inside to meet with the other tenant through the back door, the place reeks of dope, and I look down the stairs and see the two "kids" have set up shop on top of the other tenants washing machine.

She’ll be doing repairs in her own unit and would like to stay in mine while that happens since my unit is on the first floor and she can’t take stairs.

Also need one for retiling bathroom (vapor barrier and cement board).

What you are probably smelling is the cement.

Tiny kitchens and bedrooms, narrow hallways and ladder-steep stairs are also common in older homes and can be bad. 

Their database covers over 40,000 zip codes and 4,000 design and material options, providing detailed and reliable results that take into account the specificities of your project location and design choices​.Omni Calculator offers a range of construction calculators for specific needs, such as concrete requirements for various parts of a construction project (slabs, blocks, columns, stairs, etc.), as well as calculators for other materials and project aspects.

It can be placed on a cement slab too.