Do you know if there was a permit of any kind for that structure? Was it permitted as shed or garage or barn or something? Or was it built with no oversight?
If it were built as garage or barn, then that's one step toward giving it some sort of value. If built by "handymen" without permits of any kind, that's another thing entirely due to the safety issues if nothing else.
For example, I did an FHA appraisal on a property many years ago where the attached 1-car garage had been converted into 2 bedrooms... no permits. Later, a 2-car detached garage was built with side drive, and then was converted to ADU. As if that's not enough, there was an 8X10 wood "shed" back there too with an electrical line running to it. I suspect the Realtor/owner expected a male appraiser. Dressed in low-cut blouse hanging on the tips of her "hills", she blocked the door, "Nothing to see here; just storage". (I was not impressed). I investigated anyhow. Turns out they had rented that out too, and when I opened the door, there was a guy in his bed that took up virtually all the shed, watching his TV in there. Where's the bathroom, I asked. Well, the owner put an outside door to the 2nd bathroom in the main house (closing off main house access), which had wildly uneven outdoor steps. Er... that's a problem too. Then upon inspection of the 2-car garage/ADU, there was an odd light source in the bathroom. Looking up, I could see clouds scudding by through the big hole in the ceiling/roof. The tenant said she had to repaint that bathroom every month due to the MOLD in there! All "handyman" projects. The Realtor/owner chastened me, "I'm from COMPTON, and I've got FRIENDS!" Again, I'm not impressed.
Turns out the attached garage/bedrooms had previously created an electrical house-fire due to inadequate-for-living-quarters electrical service in there. No permits/no inspections. And the mold issue in the "ADU" came to light as enforcement officers came to inspect for tenant's claims against landlord/Realtor when they tried to cut her electrical lines and lock her out of her "apartment" with their 2 pitbulls along for intimidation/enforcement support. UH-Oh! And due to the black mold which had spread by then throughout the entire ADU, LA county condemned the building, the property was lost in foreclosure so the Realtor/owner received nothing for it, and the tenant was given financial assistance plus a judgment against the owner for relocating into different housing. One of the things I found so vexing, was that the woman's 9 year old daughter and just-back-from-Iraq and still in uniform son were paying rent for that mold-infested place.
I was called as witness by Legal Aid attorneys to provide proof of the property's listing as ADU rental. While in the courtroom, I listened while the judge made the observation that defendant/Realtor/owner also had another case pending for credit card fraud. Nice.
What I learned is: permits with inspections are done for good reason, and may save your property or your health.