Setting the price of buildable land in Seattle

[Lisa Toner]

Seattle real estate experts, I'd love some feedback! 

How much does a singe family home property in Seattle increase in value if it has an ideal DADU site? One thing I've heard is that the land is typically worth 1/3 of the future price of the house. I know that DADUs are more expensive to build per square foot because they're small but still have all the demands of a full house, new sewer line, etc. Is $150-200k a reasonable amount for a piece of land that could have a cottage that sells for $650-700k?

(This is a request for feedback on how to set a price, not solicitation for a deal, so hopefully it doesn't go against the forum's rules. If it does, moderators can delete. Thanks!)