Getting a preapproval for Multi-family construction

[Vadim Yuryev]

A friend of mine is a real-estate developer/ subcontractor

He currently has an investor with private funds that owns three duplex lots and is allowing my friend to build/subcontract those three duplexes using his private funds.

He gave me a deal where I don't have to buy the land up-front or pay for the construction. He said that land and building costs would be covered by the investor until it's finished. At that point, I would secure a mortgage on the final duplex and lot all-together, including his subcontracting fee. The estimate he gave me was a good deal compared to market value.

However, before construction starts, he needs a preapproval letter from the bank to show that I am able to secure the loan and get started on building. 

How should I put in a loan application to a lender? Just asking for pre-approval on a X amount of mortgage, and then secure it once construction is all finished? Or do we actually have to get the name of the private investor and name of the subcontractor and get them on the loan approval to make it work?

I'm hoping to get pre-approved in about a week once we file our taxes.