How to value vacant lot attached to a property?

[Nils Bunde]

How do you go about assessing the value of a property that has a vacant lot included next to it? 

It's is a single-family 1,100 sq ft. home listed at $390K, 3bd/1bath with a vacant 20,000 sq ft lot adjacent. Has the potential for 4-8 units to be developed. The property last sold in 2011 for $65K. 

The house alone is not worth $390K by any stretch. 

How do you assess the value of the vacant lot? There is great potential for developing it, but I don't know how to factor in that potential, if at all?

Thank you.