A lot of construction lenders won't let you do "owner builder", where you do most of the work and just a site supervisor is hired, no GC... IndyMac is one construction lender that will still permit that though.
When it comes to your existing mortgage, an appraisal with a rent survey will be needed, whatever the market rent is determined, 75% of that will be used as rental income to offset that mortgage payment.
Prepare to put anywhere from 5-10% down on the project costs, if you own the land for longer than 12 months, then any equity in the land can be used as part of that down payment... if you've owned the land for less than 12 months then only the purchase price minus any mortgages against it can be used for the down payment.
You are going to need a qualifying debt to income ratio, as well as reserves in the bank after your down payment is put down. Each construction lender has unique guidelines & procedures, and they very well could change down the road.. so it'd be wise to seek out your construction lender first to get yourself in shape for their construction loan program.
Most construction loan programs are construction-to-perm (CTP), which mean they roll over into permanent financing once the construction is complete... that saves in terms of not having to pay twice for closing costs. The negative about that though is the permanent financing terms are often higher than what you could get if you just refinanced it into a normal permanent financing mortgage.
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