It’s normally easier to qualify for investment property than a primary home but the biggest hurdle for most investors going the conforming route is having enough funds for the down payment and closing costs.  

For Example: If you have an +800 Credit Score and looking for a conforming conventional loan with 3% down only, the loan amount can be up to $484,350.00 which is very attractive at a 3.875% interest rate fixed at 30 years. 

It may currently be an 'existing non-conforming use' (grandfathered) but any 'intensification of use' i.e. adding units, would not be allowed.

Talk to some professionals about transferring the property to yourself personally, establishing a lien with the s corp, and then rate term refinancing with favorable conforming loans.

At Refi time, if you're going with a typical lender who originates conforming loans, the bad news is that you'll have to hold on to the property for 6 mo's before you can do a 75% LTV cash-out Refi. 

One concern is that parking requirements, not the headline density limit, may ultimately be what limits your project's scope.I've recommended this elsewhere, but page 20 of this book has a clever design for a 4-unit + live/work, whose construction can be financed with a single-close conforming residential mortgage. http://www.cneinc.org/missingmiddleNote that there's residential on the back of the ground floor; some zoning codes don't like this, but without it you'll have to build an elevator, which raises costs substantially.

You may be 'existing non-conforming' (grandfathered) on the elevator standard (i.e. size or location ect), but something begin broken and never fixed doesn't make it grandfathered.

Always have done what was right to make sure property conforms to a high standard and is desirable.

Hi BP,I’m looking to acquire a non conforming duplex in Spokane Washington.

Because of the double close, no Fannie conforming lender would be able to assist us since they have to deal with the actual owner on the deed.