TL;DR
Self-employed income is where most mortgage QC defects come from. The five most common mistakes — missing depreciation add-back, treating K-1 distributions as income, ignoring 2-year trending, mishandling the 50% meals add-back, and using LLMs in the math path — are all preventable with deterministic 1084 calculators. Confer ships seven income-type calculators with no LLM in any math path: same inputs produce the same output, every time.
Why is self-employed income the #1 defect category?
Self-employed income is harder than W-2 in three ways: tax-return-based rather than pay-stub-based, requires 1084 add-back rules that vary by tax-form line item, and has natural year-over-year volatility that forces a trending judgment. Manual underwriting handles each of these fine in isolation; integrated across hundreds of files a month, the error rate compounds. Per ACES Q4 2024, income and employment defects are the largest single contributor to the industry's 1.79% critical defect rate.
The five mistakes
Why does missing depreciation add-back distort Schedule C income downward?
The mistake
Depreciation reduces taxable income on Schedule C but is a non-cash expense. Fannie Mae 1084 explicitly adds it back. Underwriters who skip the add-back understate the borrower's qualifying income and decline loans that should approve, or down-tier loans into higher-priced products.
Cost
False decline: lost deal, possible fair-lending concern. Down-tier: borrower pays a higher rate than warranted, lender exposure on equal-credit-opportunity claims.
The fix
Deterministic 1084 calculator that applies depreciation add-back automatically from Schedule C Line 13 and Form 4562 detail. The calculator should also handle amortization (Line 13; different reporting), business use of home (Form 8829), and the 50% meals deduction with a single rule set.
Why does treating K-1 distributions as income overstate it?
The mistake
Distributions on a K-1 reflect cash received but not earned income for qualifying purposes. Fannie Mae requires that K-1 income be limited to the lesser of distributions and ordinary business income, with additional tests on liquidity if distributions exceed ordinary income. Underwriters who use distributions alone overstate income and originate loans the borrower cannot actually support from operating cash flow.
Cost
Investor repurchase demand on income misrepresentation: $15K–$50K per loan. ACES Q4 2024 critical defect rate is 1.79% industry average — income/employment is the largest contributor.
The fix
K-1 calculator that explicitly evaluates ordinary business income (Line 1), guaranteed payments (Line 4), and distributions (M-2 reconciliation), then applies the lesser-of test with liquidity backstop. The math is deterministic; the same K-1 produces the same qualifying income every time.
Why does ignoring 2-year trending hide a declining-income risk?
The mistake
Self-employed income volatility is normal. A single year shows a snapshot; a trend shows direction. Fannie Mae requires 2-year trending analysis with specific decision rules when income declines more than 10% year-over-year. Underwriters who use only the most recent year miss the warning signal.
Cost
Default risk on declining-income borrowers is materially higher. Investor repurchase demand on income trending defects: same $15K–$50K range as above.
The fix
Income calculator that automatically computes trailing-2-year income, year-over-year delta, and a declining-income flag when delta exceeds 10%. The flag does not auto-decline; it routes to a defined review path with documented rationale.
Why does forgetting the 50% meals deduction add-back miss qualifying income?
The mistake
Tax law allows a 50% deduction for business meals. The other 50% is a real expense to the business but represents cash the borrower spent that's not available for debt service. Fannie 1084 add-back rules treat the 50% disallowed portion correctly. Underwriters who add back the full meals expense overstate income; underwriters who add back nothing understate it.
Cost
Mostly understatement, leading to false declines. Less common: overstatement, leading to repurchase exposure.
The fix
Deterministic 1084 logic that pulls Schedule C Line 24b (meals) and applies the correct 50% add-back. Rule encoded once in the calculator, applied identically across every loan.
Why does an LLM in the income calculation path create reproducibility problems?
The mistake
Some modern mortgage automation platforms route income calculation through an LLM. Same inputs can produce different outputs due to model temperature, retraining, and prompt drift. When a regulator or investor asks why this loan calculated $84,250 in qualifying income, 'because the LLM said so' is not an answer.
Cost
Audit findings on reproducibility. Investor and regulator skepticism toward AI-generated income figures. Lender exposure on math the lender cannot reproduce on demand.
The fix
Keep math deterministic. Use deterministic calculators for the 1084 worksheet and every income type. LLMs are appropriate for document classification and natural-language explanation; they are not appropriate for the math path. Confer's pattern: 7 deterministic calculators (W-2, Schedule C, Schedule E, K-1, investment, retirement, other), no LLM in any of them.