Luxembourg Indépendant — Form 100 & CCSS Return

Establish the client's tax-resident status, CCSS registration as an indépendant, VAT position (registered above EUR 50,000 or franchise scheme below), and tax class (classe d'impôt 1, 1a, or 2). Confirm the income year in scope and whether any quarterly advance payments (avances trimestrielles) have been made. Without these facts the correct scale, contribution rate, and VAT treatment cannot be determined.

Classify every transaction on the client's bank statement(s) for the income year. Identify gross professional income (bénéfice commercial / bénéfice provenant de l'exercice d'une profession libérale), strip out VAT collected on sales if registered, and sort debit transactions into fully deductible operating expenses, restricted items (representation costs, mixed-use), capital items requiring depreciation (amortissement), social contributions (dépenses spéciales — not operating expenses), and excluded items (internal transfers, VAT payments, tax advance payments). Flag all Tier 2 items (home office, vehicle business-use percentage, capital depreciation rates) for reviewer sign-off.

Compute CCSS social contributions for the self-employed person. The indépendant bears both shares of pension (16%), health benefits-in-kind (5.60%), and health cash benefits (0.50%), plus dependency insurance (1.40% on income less the EUR 675.94/month abatement, uncapped). The combined rate is approximately 23.5–24.5% of net professional income. Contributions are capped at the monthly contribution ceiling (EUR 13,518.68 from 1 May 2025). Social contributions are recorded as a dépense spéciale, not as an operating expense, and reduce taxable income at the Form 100 level.

Apply all personal deductions and abatements available to the self-employed client beyond social contributions. These include: third-pillar private pension premiums (deductible up to EUR 3,200 for 2025, rising to EUR 4,500 from 2026), personal loan and insurance interest/premiums (up to EUR 672 plus EUR 672 per dependent), mortgage interest on the principal residence (ceiling-based depending on year of purchase), home-office proportion, and the joint professional abatement (EUR 4,500 if both spouses/partners have professional income under Class 2). Confirm all supporting documentation is available.

Apply the 2025 progressive barème (23 bands, 0%–42%) to the net taxable income (revenu imposable) after all deductions and abatements. Add the employment-fund solidarity surcharge (7% of PIT for income up to EUR 150,000 in Class 1/1a; 9% above that threshold). Reconcile the tax computed against quarterly advance payments already made (avances trimestrielles due 10 March, 10 June, 10 September, 10 December). Compute the balance due or refund expected on the final assessment (bulletin d'impôt).

Populate Form 100 (modèle 100) on MyGuichet.lu with all computed figures: professional income (bénéfice), operating expenses, social contributions (dépenses spéciales), other deductions and abatements, tax class, and advance payments. Cross-check every input line against the working papers from prior phases. The filing deadline for income year 2025 is 31 December 2026 (window opens 7 April 2026). Late filing attracts a surcharge of up to 10% of tax due, plus a potential fine of up to EUR 25,000, and late-payment interest of 0.6% per month.