A standalone ADU is almost always going to cost more than doing an addition, so the value added or cash flow increases will have to justify each.I work with a team of investors who will buy SFR homes then build a new ADU on the lot and add a separate drive way and meters for utilities in North Texas. 

The hot tub was just for the luxury of having it, but we justified the purchase by increases the AirBnb nightly rent.

I do worry a bit that the market could shift downwards, but I think I justified it to myself by saying 'in an economic recession, people will still travel, they just won't want to pay $800 a night to stay at the Grand Wailea.'  

Fees should be clearly stated in writing, easy to understand, and justifiable.

Hey @Justin Woods - as far as I'm aware, the property has to be a complete investment property to even qualify for cost segregation studies and on top of that it has to be large enough to actually justify the study costs/benefits.I don't think it's worth it for a SFH househack you're turning into a 2 family. 

Some things do not justify that hassle.

Mold remediation can be expensive, but it may not be big enough to justify an insurance claim.

It is very important that you solidify comparable sales in the area to justify the higher value and request a reconsideration of value if this is possible with your lender. 

That property has tenants which would cover the monthly payments at its current rent which is well below market value but the property would need love to justify an increase in rent.Very grateful to have made this much progress in the span of less than a year and a half and currently just trying to make sure I have all of my ducks in a row to ensure I can build these three properties into a stable few investments.Looking forward to the future,Austin

If separate water line services for each unit weren't part of your original scope of service, then they are justified in asking for additional payment to redo the plans.