They suggested adding an external cleanout for about $8,000, and we agreed to split the cost by adjusting the home’s price.However, after installing the cleanout, we discovered that the actual problem was far more severe.

Here's what the quote covers:- Conversion of 2 bedroom, 1 bath main house to 3 bedroom, 2 bath (~1040 sf footprint)- Demolition of existing walls (existing dining room wall, wall between kitchen and living, bathroom closet)- Finished Shell (Reframing opening to living room and kitchen, shifting openings for bathroom/bedroom, new walls at bathroom and dining, level - Replace all aluminum windows with fiberglass windows and screens- Upsizing plumbing, and relocating fixture locations as necessary- Upgrading electrical panels- Assume heat pump and ductless mini-split for heating and cooling- An allowance for kitchen appliances including combination range, hood, dishwasher, microwave, refrigerator, and washer/dryer- An allowance for mid-grade plumbing and lighting fixtures (Delta, Kohler, or equivalent)- Mid-grade plumbing and lighting fixtures (Delta, Kohler, or equivalent)- Ikea cabinets or equivalent with solid surface countertops, includes island- Assume solar system to be installed on the house including battery storageFor the garage ADU, the garage would need to be demoed and rebuilt, but we would benefit from building within the setback since the garage is on the property lines.

They are basing it off a depreciated cost from an original install price of $1440 and claiming a 7 year life cycle.

These are super easy to install.

But they did so with the request of the seller buying and installing new washer and dryer in the house. 

In my county we have a homestead tax credit for owner occupied houses which limits your property tax increase AND you can pay your taxes in 2 installments .

We've also privately lent with similar terms for our new installs.

Significant defects in the roof and siding resulting from improper handling of materials, and improper installation. 

Who is paying for the installation?

You could start an HVAC company that does installs and/or does trouble shooting when it's peak season.