(And even if they think they have a good eye.) 

But even that depend what your end goal is.

But, in this case, I am assuming $45k will not even cover the material costs of a whole house, let alone any contracted labor.

Even if you use a VA loan, you are using your credit.  

Get your architect involved as early as possible - even before the land is purchased….

The house is sitting on the market under the R-2 Zoning code as a single-family house not as 2 o 3 units.

You need to find single family with ADU that sold in your planned purchase tract that appraises high enough to make this work.

In fact, it’s even easier to justify it with short-term rentals because if you indicate that you plan to rent it out to someone long-term, that would obviously prevent you from being able to use it as a second home. 

I am new to this but I really need to start investing into my future and single family house for us isn't doing it. 

I bought move in ready Class C Indy in 2023 - it’s been a headache even though price points are low.