Super tiny SFR In Monrovia - need help

[Mikhail Petrenko]

Hi everyone!

My plumber wants to sell 1bd 1 ba 622 SF house in Monrovia CA (Mayflower/Foothill) as is.  The house is dilapidated but has a large 6500 SF lot (zoned as a duplex). His uncle still lives there (does not pay rent and does not want to move).
I may build 2 SFRs and remodel the existing house and use it as ADU.

It is almost impossible to find local comps (such a small house).

What would be a fair offer from an investor's standpoint for such a deal?