How to calculate Value adds

[LaJoy Toney]

in terms of multi family/ house hacking. When adding value to the dwelling such as W/D, Dishwasher, new roof, extra room(s) ; How to you calculate those numbers into a plan? 

Do you literally add what you paid for it? I.e W/D cost $1300 so you add $1300 to what the property is worth at the moment?

Or is it more you look and compare to other similar dwellings in close proximity to yours and you go off of demographic data?

for example: 

You look on CL and see a property w/o such amenities compared to properties with similar amenities and go off that?

Thanks for your time :) I’m super green  so break it down Barney style :)