The project, not your pay stubs
A fix-and-flip loan is a business-purpose loan on a property you will not live in, and business-purpose credit sits outside the federal rule that makes a lender verify a borrower's ability to repay a home loan. That is why it can be qualified on the property, the budget and the plan rather than on tax returns. It is not a loophole: your credit is pulled, the lender asks what you have finished before and what you have in the bank, and the moment you intend to live in the property the loan is a consumer loan with every rule a home loan carries.
The numbers have to work
The appraiser estimates the after-repair value from the plans and from sales of comparable homes that have already been renovated, and the lender sets how much of that value and of the cost it will advance. Against it go the purchase price, the rehab budget, the interest you carry during the project, the points and fees, and the cost of selling. What is left is the margin, and it has to survive a surprise behind a wall and a slower sale than you planned.
Your own money in the deal
The lender does not finance everything. You fund the gap between the purchase price and what it advances, the closing costs, the interest payments while the work is under way, and the work itself between draws, since a draw may reimburse a stage only after it has been inspected. A lender may also want to see cash left after closing, not only cash at closing.
Experience
How many projects you have finished, and how they went, can change the terms a lender offers and how much of the project it will advance. We arrange fix-and-flip loans for a first project too, and on one, expect more asked of you, a closer look at your contractor and your budget, and a harder question about what happens if the sale is slow. A record of finished projects is what makes the next loan simpler, which is a reason to keep the first one modest.
Where it is not the answer
A home you will live in. That is a consumer loan, the lender must verify your ability to repay it, and the right route is FHA's 203(k) or a conventional renovation loan such as Fannie Mae's HomeStyle Renovation or Freddie Mac's CHOICERenovation, each of which finances the purchase and the work together on a home you occupy. A property you mean to keep and rent is a DSCR loan, from the start or through a refinance once the work is done. And a project that only works if nothing goes wrong is not a loan problem; it is a project to walk away from.