For instance one reason a property can have a higher price is it has work done to it. 

Displacement of tenants - Let's say for instance that your tenant who's boyfriend has appeared to move in, does actually live there and you are inadvertently accepting funds from the boyfriend.

If you include the rental income it would in this instance, but what if I did not have any other proof of income?

not necessary in most instances..

For instance, a landlord I know had a unit at first was going to be offered as is for $1800 but after a $45K renovation upped it to $2500.So, where I'm going with this is: is it fair to also increase the rent if a tenant requests the unit/building to be deleaded?

Again... some of the numbers required guestimation- I am using the average of my rental income comps for instance.

However, there is not much you can do, for instance, recently, when a teenage son of one of your long-term tenants decides to punch holes in walls and kick in the front door. 

For instance, if you build an ADU for $150,000, I haven't seen the value go up much higher than $150,000 for the property. 

For instance how you collect rent.

@jered sturm, very interesting prospective, and i certainly see and feel what you are  saying, and makes me wonder how folks will respond, for instance, any trade that can be reduced in new construction, like using sips wall systems instead of custom framing, or a trend toward modular, etc.