Yes we avoid custom work to help keep cost down. 

FlooringScope of Work:Replace carpet in bedrooms with engineered hardwoodRefinish existing hardwood floors in common areasInstall durable tile in wet areas (bathrooms, laundry room)Estimated Cost: $20,0004.

Figuring out the true cost will heavily depend on the type of dig down, depth, and total square footage. 

On your tax return you would report the proceeds, subtract costs and proceeds paid to seller, then the remaining profit would be your flat fee.

Does the cost per sqft account for electric and plumbing? 

I'm looking at monthly cost to seller vs. anticipated rental income, purchase price relevant to the ARV of the area and interest rate, balloon, etc.

Here is an example of what it costs you to avoid professional advice.

It will cost less than $150,000 and will need moderate renovation work.

If it costs $100,000 to install an ADU and it rents for $1,000 a month, then it may be worth it.

I already have some equity in the property after doing a complete rehab but I don't want to diminish my returns selling it after one year, paying for commissions/closing costs.