Plan for complete rehab cost including, cot associated with older homes, permitting, and the like.

Ronesha,I'd need more information to give you an accurate answer for your specific property/situation, but, speaking generally, first you would apply for a zoning permit with L&I.

Try contacting the municipality code enforcement or licensing/permit departments for a list of condemned properties. 

Seller never got a permit replace a beam in the basement.

The house is zoned commercial which is rare and means that I can get the permit from the city. 

Make sure you comply with all local zoning and permitting for duplex.

Question: My seller just received a contract offer from a buyer who used a Florida Realtors/Florida Bar “AS IS” Residential Contract for Sale and Purchase, and they noticed that the buyer’s offer included a provision in Section 7, Assignability, that would permit the buyer to assign the contract.What are the ramifications if my seller accepts this assignable contract?

It looks like they converted the SFH into a duplex without pulling permits to do so.

And my property is able to build a multi-family house with permits and plans required.

Sounds like a solid plan just make sure to talk to the building department and pull all permits before starting the renovation.