Underwriting is the lender's final, thorough review of your loan file — the step between "you're probably fine" and "here are your keys." An underwriter verifies your income, assets, credit history, and the property itself, then decides whether the loan meets the lender's rules.
Think of preapproval as a quick screening and underwriting as the full physical exam. Underwriters focus on the three C's: credit (your track record), capacity (your debt-to-income ratio), and collateral (the home's appraised value).
A concrete mechanic: underwriters issue "conditions" — requests like a letter explaining a large bank deposit or an updated pay stub. Clearing conditions quickly is the single best way to keep your closing on schedule. The classic self-sabotage is opening a new credit card or financing furniture mid-underwriting: lenders re-check credit right before closing, and new debt can shrink what you qualify for or sink the deal entirely.