[EraPartner]

Requested by EraPartner · project thread

Summary

The special social security contribution (law of 30 March 1994, "bijzondere bijdrage voor de sociale zekerheid") was evaluated on the net professional income without pensions. Art. 107, 2° of that law defines the household income as each member's total net income of WIB 92 arts 7-116 (every income category, after the arts 104-116 deductions such as alimony), without the art. 171 separately taxed income, less the art. 34 pensions. Source read on Justel: https://www.ejustice.just.fgov.be/eli/wet/1994/03/30/1994021117/justel

Consequences fixed:

• Other taxable income (rental, …) now enters the base. Example IY 2025, single employee 50,000 gross plus 10,000 other income: engine gave 343.16, law gives 566.93.
• The alimony deduction now lowers the base. Same employee with 10,000 alimony paid: engine gave 343.16, law gives 246.98.
• Under a joint assessment the alimony balance charged on the other spouse (art. 105, 3°) follows the same allocation as the taxable income; pensions are subtracted from the household total before the clamp.

Only households with an employee or civil servant and either other taxable income or alimony paid are affected; everything else is unchanged (all existing scenario-matrix expectations pass without edits). Found by the 2026-10-29 source audit (vision notes, queries/2026-10-29-belgian-tax-source-audit.md).

Type of change

• fix — bug fix
• feat — new feature
• refactor — no behavior change
• perf — performance
• docs — documentation only
• chore / ci / test — tooling, build, or tests

Checklist

• Tests pass (bun vitest run in packages/shared-utils: 418 pass) and lint is clean (bun run lint:projects, bun run lint:dead)
• Types check (tsc --noEmit in packages/shared-utils)
• Conventional Commit title (type(scope): subject)
• No leftover console.log / debug code
• Affected docs/ pages updated (docs/features/belgian-tax.md, docs/reference/belgian-tax-rule-matrix.md)
• i18n: no new strings

Architecture & data

• No architectural change
• No API change (the csss.base figure changes meaning per the law; the shape is unchanged)
• No schema change

Verification

• packages/shared-utils: bun vitest run → 23 files, 418 tests pass, including the parity property test between the legacy computeBelgianPIT and computeHouseholdTax.
• pit.test.ts: the two tests that pinned the old base ("uses professional income only …") now assert the statutory base; household.test.ts gains three cases (other income, alimony overflow under a joint assessment, pensioner paying alimony).
• tsc --noEmit, bun run lint:projects, bun run lint:dead: clean.

Residual risk & follow-ups

• Separately taxed income (art. 171) and treaty-exempt income (arts 155-156) are not modelled, so they cannot be excluded from the base; the model has no such inputs.
• The csss_mixed_activity_base_unverified issue is left as is; art. 107, 2° suggests the whole net income enters the base regardless of activity kind, which could close matrix open item 10 in a follow-up.

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Generated by Claude Code

[github-actions]

Coverage Report for backend

Status	Category	Percentage	Covered / Total
🟢	Lines	91.06% (🎯 80%)	23170 / 25443
🟢	Statements	89.33% (🎯 78%)	24933 / 27910
🟢	Functions	91.44% (🎯 76%)	5004 / 5472
🟢	Branches	81.61% (🎯 68%)	17481 / 21419

File Coverage

No changed files found.

Generated in workflow #1958 for commit f13938a by the Vitest Coverage Report Action