Developing Commercial zoned property in seattle

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I have a really good lead on NC 2 -40 zoned property in seattle that already has a duplex and a 600 sq ft commercial building being leased. The owner is taking his time on selling but seems to be ready, he said he brings in about $3750 in rents and owes nothing. I want to purchase and develope 12 four story live work townhomes with 4 three story townhomes, selling now at around $399,000 a piece will be more when ready to sell. Can anyone help me out I know the zoning and what can be built already gone through that, looking for answers on how to write the deal and all other options. Thanks in advance.