The third and most expensive option is adding on an addition, but this has the benefit of increasing the size of the house while maintaining covered parking.Keep in mind with option A that you need to be aware of the existing square footage.

Those aren’t fair comparisons.You should compare 10 quadplexes to 40 SFHs.They both have their pros and cons.10 quads will usually be less square footage than 40 SFH’s.

If you're patient and willing to do a lot of research, you might be able to find a motivated seller and get something below MV, but they'll be pretty rare.To get a better idea of what the MV is for the neighborhood you are looking at, have your realtor run some comps for the bedrooms and square footage you're looking for.

That would at least give you the sale price and date but I wouldn't count on it for correct information about the property, like square footage, beds & baths

Make sure the comparable's are within +/- 20% of the square footage of the house you are looking into. 

The wholesaler told me that permits were not required because the layout was not altered (just switch out sliding doors with a wall) and that no actually square footage were added.

Given the basic requirements, any lot that has sufficient space to physically house an ADU likely is legal to build on, and lots that have existing multistory houses tend to be best (as the lot coverage on a multi story house will be lower than the lot coverage on a single family house with the same square footage).

Deduct the rehab costs from the new square footage cost and deduct another 10% for an offer, if you really think it is worth your effort.

Awesome job with adding a bedroom and sq footage!

My acquisitions agent has an excel sheet that auto generates estimated repair costs solely based on square footage.