I have a pretty decent monthly net income but plan on increasing it further by downsizing from a 1 bedroom/1 bathroom apartment to a studio (I've realized I barely use the living room area during the week anyway, so much of my current space goes unused).

buy "Tax Free Wealth" audio book by Tom Wheelwright.

Theoretically, you can do this with any other type of content, including podcast or audio files.

The only way I see this being worth it long term (and Ive put a lot of thought into this) is to lay a slab and get a used single wide and convert into two studio apartments for a total cost of <$15k.

You can get a lot of free audio and pdf stuff from a free download at limewire.com.

First, would love to know if anyone else has done this and any advice you'd give, and secondly what you all think about my high level calculations:- $80k to build (likely thru HELOC/refi) - Is this estimate in the right ballpark for a small studio/loft ADU?

@Julie Marquez I've also seen a few homes that have either studios in the backyard or the basement is detached from the upstairs.

If a studio rents in your area for 600 a month,, I'd ask  him to pay 450 iutilities included. 

Are they mostly smaller spaces like studios to 1BR, or do you also have 2-3 BR rentals?  

We purchased a building for $11,500 that has two one bedroom apartments and one studio, but the city zoned it as a two unit building so we cannot rent out the studio.