I am seeking applicable first steps as to how to turn my primary residence SFH into a Co-living space. 

I am now trying to get into a fourplex which my mother will occupy - she will be in the title and I will be a co-borrower on the mortgage.

I was wondering if it is legal to have a sibling do a mortgage application with me and my wife as a "co-borrower"  as a primary resident and use that to our advantage for lower rates and down payments for a rental.

I am looking for advice on how to find people that would be interested in creating a partnership and using a co-hosting service.

Even cash Buyers have some common sense and have to recognize that, unless they are tearing it down to do a rebuild, that somewhere along the way their capital is at risk and would not get involved in it.If I were you, I'd just say I can't help you with this one and move on.To answer your question though, if it is zoned as a SFR, if it has a CO as a SFR, appraises as a SFR, has a utility set-up as a SFR, then one should value based on the Sales Comparable Approach and not an Income Approach. 

In NYC where I am, the finance department had me down as a triplex and the  buildings department has me down as a CO is for 2, after they discovered an illegal unit.

They expect higher net worth and some more experience, but if you're able to find a co-sponsor that meets those requirements, that would be another option. - Tim

It's not only about credit score and down payment, they are assessing your ongoing ability to pay given your other required payments.It could be perfectly reasonable if you hack it with some roommates, duplex maybe, and/or get a co-signer. 

possible to  line up a  co borrower  ?

As I live in a Coop I am convinced that I do not have a CO (I only own shares of the building)...