The JADU can be fully built within the envelope of the primary structure, so doesn't necessarily have to look much different than a single family home from the exterior viewpoint (ignoring the detached ADU).

Get your mind right and there’s no obstacles.

With a two car detached garage.It’s a 5 minute walk to the beach and brand new everything, property is in perfect condition.Asking: $114,000Taxes:$2,350Rent:$600Rent:$800Tenants pay utilities.Mortgage would be around $600 with taxes and insurance and 25% down.That would leave me with a cushion of around $800 pay for vacancies repairs or other costs.

I bought a single family house with a detached apartment for $250,000.

My biggest obstacle is my wife,  she gets irritated when she sees me researching anything that would be considered entrepreneurial.  

The biggest obstacle in my mind is to try not to repeat your mistakes and learn from them.

You're aware that there are multiple ways to get a return on that money without the obstacle or inconveniences of $40,000+ down for a down payment, moving, or self managing a rental.

There was a deteriorating detached shed in the back yard of one of my places. 

How do you split the water bill among 2 tenants in a SFR property (main house and detached unit)?

I would find out what your local building code is for stairways, step width, stair tread, riser height and incline,, just as you think.. this is complicated. just because it's grandfathered in doesn't make it less of a obstacle just makes it a legal obstacle that someone can always find a lawyer to help in filing a suit..