Sometimes my first thoughts are so different than the crowd's that I wonder if I've been working by myself too long!
"Do I require that the garage be converted back to its intended use or just ignore the conversion and give it no value?"
Is your assignment to form an opinion of value, or is it to physically change the subject property so that it is easier to appraise (joking)? Appraisers frequently speak of "giving value" to something, but all we can really do is either make a hypothetical assumption and treat the property as if it were different than it really is, or we can include the amenity in our opinion of value.
In this type of situation, I would compare the property to other duplexes, with the converted garage treated as a separate amenity. You may decide that the market pays a premium for properties with converted garages, or the market might be neutral on the matter, or the market might demand a discount to pay for future re-conversion back to a garage. Any of these positions could be reasonable, based on the market's reaction, which is bound to be different even in different parts of Reno. Hopefully, there are a couple of sales with similar conversions. If not, you're going to have to get creative.
In terms of estimating value, the actions of the market could be more important than what the planners say. I've seen garage conversions that the market will pay a huge premium for, even though they're illegal and not built to code. It would be misleading to state that a conversion like that has no value, if the market clearly recognizes it. On the other hand, if the market demands a discount for properties with converted garages, it would be misleading if the conversion were just ignored.
After all that, as a convenience to the lender, I would probably point out that the conversion is separately adjusted in the Sales Comparison Analysis and I would provide a rough estimate of the cost to re-converting the garage. Then if the lender decides that they don't want to lend on the portion of value attributable to the conversion, it will be obvious what contribution it makes to the total value.