Building a carriage house for rental - Memphis

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We recently bought a home in a high income/ownership historic area where carriage/back houses are pretty common. Our house doesn't currently have a garage so we are toying with the idea of building a 2 car garage with rentable living space above. Carriage houses in Memphis Central Gardens rent for anything from $550 - $950, (depending on size) and the renter base is usually a single professional working downtown or relatively close to midtown.

Right now, we are having an architect draft plans but I'm wondering about the best way to go about financing the build. Of course, the rent would help to subsidize the build and long-term, the structure would add value to the property but what options are there other than HELOC? I don't think a 203(k) is even possible for this...

Has anyone done something similar?