While not specifically described as a form-based code, Santa Cruz and other cities like Portland already address the potential nuisance impacts from additional dwelling unit density in residential neighborhoods.
Instead of legislating how the structures can be used (although they do that too), perhaps a more effective method is to regulate the structure.
For example,
Portland requires a 60' minimum front yard setback (to ensure that the ADU is built in the rear and does not dominate the facade of the single-family dwelling), limits the max lot coverage of an accessory structure to 15% of the total lot size (for a 5,000 sq ft lot, that would be a max 750 sq ft ADU), limits the ADU floor area to max 75% of the main house, and limits ADU max height to 18'.
These may or may not be desirable in Honolulu, but what these rules attempt to do, is mitigate the physical impact vis a vis the visual appearance that ADU's might have on a neighborhood.
Santa Cruz's ADU manual has extensive graphic and textual descriptions of ADU design and siting, according to neighborhood character (post-war, traditional, transitional) and detached/attached unit configuration. While it includes notes to remind the designer to orient windows away from neighbors (if the ADU will be close to the property line). Some comments seem to be guidelines for consideration (ie. My favorite: "Of course an important part of being a good neighbor is discussing your plans with adjacent property owners."), other features are requirements.
