The duplex would also be close enough to the road to access city water and sewage, where as the rest of the property is not close enough to access them. 

Based on this post and your other post.You're firmly out of DIY waters here, and might require an amended 2018 return.

The most sure usage of an ADU is to hold it as a buy n hold that produces additional income. 

New flooring, interior and exterior paint, new roof and all new water heaters.

Most or all of of the water will be gone by the time it gets to the end which is why there is so much gravel all along the run.

You also want to make sure if you've included water and electricity based on one person living there, you account for the extra consumption-if the tenants pay water and electricity, this won't matter.

Does it have water/sewer/septic, electric, gas, etc to the lot? 

She did not use the 3rd floor or 2nd floor, as she had holes in the roof allowing rain water to further deteriorate the house.

Unless there are water lines running in a shared wall in the garage, I wouldn't spend the $90 unless you are in the deep north. 

Managing water accumulation while going thru process.