Building more units on a lot zoned commercial

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The story is I own a duplex in the Seattle area that I have decided I am going to sell in the foreseeable future. The property is zoned RM-24. I am thinking it may be worthwhile to look into developing the property to increase it's value substantially prior to sale. It's just an idea at this point and really have no idea where to go from here. This stuff is way out of my league as I only own this one property so far. I have no idea about the permits required or who I would talk to in order to see if moving forward would be worth it. Is there anybody out there that has experience adding units to a duplex? If so I'd love to hear how you did it.