If you are in a high cost of living area, such as San Francisco, you need to stay below the conforming loan limit of $970,800 to get the best rate without needing to put down 10%.

Conforming plus a second mortgage to make total payment lower.

I've dealt with my fare share of non conforming and it can get tricky make sure you have an experienced architect that knows setbacks etc...you will crush it

- I would put 25% down or whatever % you need to get the home loan amount to the conforming limit, I think it's around $453,000.

HELOC's are just a product of the individual banks that offer them, so they can essentially make up their own rules (unlike conventional conforming mortgages that have to conform to the guidelines established by Fannie Mae and Freddie Mac).I have a couple HELOC's myself on investment properties, and in doing my research I found that most banks want you to be on title for at least 6 months before they'll do a HELOC. 

It is curious, I thought you could get a conforming mortgage.As foreign person, you can try HSBC mortgage, otherwise US micro-banks (the smaller, the better) have portfolio loan department and will lend to foreigners.Cash on cash return is an affordability and solvency measure.

As long as there is sufficient parking, and lot size to house the structure and conform to impervious ground and FAR calculations. 

I recently closed on a 4 unit and there's a non-conforming 5th unit with drop ceilings.

Cash out refi'd for 80% of final appraised value ~105k (total project cost was ~85K) closed about three months after we acquired the property and noone asked where the funds came from to purchase the house although our strategy was disclosed to the loan originator and the VP of Mortgage Lending of that credit union (It's a non conforming lender) prior to our application. 

I would assume they were legal when they were built, and grandfathered in when it was zoned single family....grandfathered non conforming properties do have some restrictions, involving major alterations, etc.If the properties are large enough, the lots could be reconfigured to have two different, compliant properties, assuming you could meet current set back and lot size requirements.