You can't fit a square peg in a round hole. Because they are still unusual in my area most of the form appraisers will not accept the assignment because of the time and effort required. They would rather turn and burn. I have done three tiny (<600 SF) houses, all on multiple acres, in the last couple of months.
I found the cost approach to be my friend, and this is one that will need to have a very strong cost report opinion of value. Sounds like you can nail down the land value fairly easily. Now look at the cost of construction for the size and type of dwelling located on the property. One big question are the solar panels and are they real or personal property. Myself I would include their cost in the report, but then discount them based on acceptance. You can probably obtain that figure from other sales of grid-based properties vs solar homes, whihc are similar in size. Then also consider the cost of an onsite LP fired generator, which depending on the area may be more reliable than solar and the cost of acquisition and installation may be less. True that solar is "free" energy, but it would be interesting to amortize the cost difference (including operations and maintenance) between the two over their expected life span to get a feel for the true cost/savings. Additionally, you should consider the cost to provide grid-based service to the subject property. While expensive it just might increase the property value enough to make it worthwhile, especially if the current or a future owner may want to increase the living space or add additional structures which may require more power than solar can provide.
As to being a tiny home, which sounds like it was built off site and dragged on to some sort of foundation. You didn't create the problem you are just trying to value it. Like all comparable searches look for other dwellings that are as similar as possible and then work on match pairs in an effort to come up with adjustment factors. When it comes to the difference in construction you can go back to the cost approach. Many times, in commercial work when comparables are few and far between we end up comparing block, to pole, to steel frame construction. We do the best we can to make market-based adjustments, but sometimes you just have to say the adjustment is something like 50% of the cost difference to construct new.
Gather as much information as you can, explain the situation and the efforts you have gone to, explain in your report all of the steps used to arrive at your various adjustments and values and write a good summation. I have always worked under the premise that if the client or reader of the report agrees with the steps I have taken, and the methodology used, they have to agree with my conclusion.
Nothing is impossible, it just takes a little longer sometimes.
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