How much value does turning duplex to a triplex add?

[Harman N.]

Hey folks,

Roughly how much value does adding a legal ADU to a duplex and converting it to a triplex bring?

The duplex is in the Inner Sunset in San Francisco, and the 2 existing units are 2BR/1BA 1200-1300 square feet. The additional unit would be a ~450 square foot 1BR or studio. 

It seems like the average price per rent-controlled unit in SF is around $400K. That's the number I've seen in some reports, and also what I calculated by going through a couple of months of SFAA (San Francisco Apartment Association) magazines and writing down the sale price and # units of multi-family buildings. 

But of course that's just an overall average, and wouldn't necessarily be what applies in this situation. So was hoping someone had guidance