If a builder does it for me he has offered to do it for $85.00 sq ft.....850 sq ft x 4 apts=$289,000.When the 4 plex is finished it will be worth $350-400k....I'm currently being offered a 3.37% APR on a cash out on my home on a fixed rate conventional loan. 

Bank conventional loan How did you add value to the deal?

Conventional Did you work with any real estate professionals (agents, lenders, etc.) that you'd recommend to others?

25% down conventional.

We would agree on a price, and I would send the contract via DocuSign for the Seller to digitally sign.

It will need Cap-Ex sooner or later, so BRRRR would be the ideal approach here; I'm approved for a conventional loan, but could start applying in earnest for a bank loan, or possibly pursue an option I have to partner up to make a cash offer (this deal is right around that 100k mark where my lender would only reluctantly do such a small conventional loan...another reason BRRRR is appealing).

I originally applied for a 30-yr fixed conventional loan for a single-family.

SFR are ok to overpay a bit right now depending on the strategy IMO but people should be worried about overpaying for residential multi with non primary conventional or commercial loans.

If you are using conventional you will need 20% of that.

I have been doing my research I wanted to tap the community about non conventional financing ideas.I am aware I can refinance the houses and use the money as a down payment on the building.