FYI we've changed our approach entirely and have lower fee initial entry points to avoid the challenges of long term project based arrangements so we can catch issues much sooner.

You can make certain assumptions about a property based on photos, permit records, and year built.

Your P/I is based on the amortization period during the balloon term. 

If I were you, I'd play out a few different scenarios.1.

@Jordan Myer she is asking to lower it base on market on the area since it's been declining.

Also, some base their offer price on the repair price (i.e., $50/sq ft - seems too low), which makes no sense to me.

I'm based in Virginia.

Add $800 for paint, floor covering and essentials plus my laborer for 2 days and my total cost was about $1400.I painted the 30 1200 to 2700 sq ft homes in Las Vegas with two helpers including ceilings and it never took more than two days to paint a house and that included removing all the electric wall plates, dropping ceiling light fixtures, removing smoke detectors, cleaning windows, installing new blinds, cleaning bathroom fixtures and it never took the 3 of us more than 2 days because I am good at knowing how to get the most from my workers like a Lakers coach knows which players to put on the court because the cost to do a paint job is not as critical as the time it takes because when I go to Las Vegas to manage a job I have to leave Los Angeles and my 20+ employees without my micro-management and "When The Cat Is Away The Mouse Will Play" if you know what I mean.I am a very difficult person to work for because I am 71-years old, a workaholic, a fast worker and when young workers are slow I fire them within minutes when they are working because I don't like to have to waste my time and stop the work I am doing to constantly check on my workers and I don't like to get emotional and have to keep correcting workers when they work sloppy and slow.

Several methods come to mind for the ADU.   1) a regular or periodically adjusted flat fee (based on an estimated usage); 2) factored into the rent (essentially the same as a flat fee); 3) percentage proportionally based on sqft. 

Its super easy if the home had a pre-existing unit - just take these specs and plug and play into new equip.