Corporate tax return in Luxembourg: form 500, MyGuichet and deadlines

In short. A company's tax return in Luxembourg goes through a single form, form 500, which covers corporate income tax (CIT), municipal business tax (MBT) and net wealth tax. For a resident capital company the filing is electronic and mandatory, through MyGuichet.lu and a LuxTrust product. For tax year 2025, the forms have been available since 7 April 2026 and the submission window runs until 31 December 2026. Here is how it all fits together, without the jargon.

Every spring, the same question comes up for SME owners: what does my company file, where, and by when? The corporate tax return in Luxembourg has a reputation for being technical. On substance it is, but its administrative mechanics come down to a handful of clear rules. This guide walks through form 500, the mandatory online filing and the current campaign's deadlines.

What is the corporate tax return (form 500)?

In Luxembourg, a capital company does not file one return per tax. It files a single return, form 500, the "return for corporate income tax and municipal business tax of resident undertakings". That one form is the basis for three taxes at once:

• corporate income tax (CIT), calculated on the taxable profit for the year;
• municipal business tax (MBT), based on the same profit but at a rate that depends on the municipality of the registered seat;
• net wealth tax, calculated on the company's net assets as at 1 January.

We covered how these three taxes are calculated, their rates and reliefs, in our article on corporate tax in Luxembourg. The point to keep here is simple: one financial year, one return, three taxes. This is the return your accounting firm prepares from your annual accounts.

Filing is electronic and mandatory

This is the point that still catches out owners used to paper. Since tax year 2017, electronic filing of form 500 has been mandatory for resident commercial undertakings, as the Luxembourg Inland Revenue (ACD) sets out. This covers capital companies: the private limited company (Sàrl) and its simplified form (Sàrl-S), the public limited company (SA), the simplified joint-stock company (SAS), the corporate partnership limited by shares (SCA) and the European company (SE).

In practice, the procedure is done in the professional area of MyGuichet.lu and requires a LuxTrust authentication product (token, Smartcard or app). Paper filing is no longer an option for these companies. One useful clarification to avoid the acronym soup: the annual tax return goes through MyGuichet, whereas your VAT returns and the filing of the financial data from your annual accounts go through the eCDF platform. Two channels, two logics, often mixed up.

The 2026 campaign deadlines

The return covers the year just ended. So in 2026 you file the return for tax year 2025. Each year the tax authority makes the forms and online services available on a set date, then sets a submission window.

These are the dates published by the authority for the current campaign; the ACD tax calendar is the reference and is worth checking each year, as it shifts by a few days. A method tip rather than a race to the deadline: aiming to file well before December, once the annual accounts are closed, avoids the year-end bottleneck and leaves time to plan for the tax due.

Annual accounts first, return second

The tax return is not filled in a vacuum. It flows from your annual accounts: taxable profit starts from the accounting result, adjusted for tax rules (non-deductible expenses, depreciation, loss carry-forwards). In other words, an accurate return assumes accounts kept properly all year, not reconstructed in a rush at filing time.

That is also why the tax return and the filing of annual accounts with the trade register move in parallel, each with its own deadlines. An SME that lets its bookkeeping slip ends up handling both at once, late, with the risk of errors and surcharges. Up-to-date accounts turn the return into a checking formality rather than a construction site.

The mistakes that cost the most

On this topic, the nasty surprises rarely come from the calculation itself. They come from logistics. Three come up regularly.

The forgotten LuxTrust certificate. Many owners discover on the day that their LuxTrust product has expired, or that they never activated the company's professional MyGuichet area. Sorting it out takes days, sometimes more. This is prepared upfront, not the night before the deadline.

Confusing the channels. Filing your financial data on eCDF does not replace the tax return on MyGuichet, and vice versa. These are two separate obligations, on two platforms.

The silent delay. A return not filed on time exposes the company to an estimated assessment by the authority, set on a basis that is rarely in your favour, plus possible surcharges. The point is not to "buy time" by putting it off, but to keep control of your figures.

How we handle it at Advena

We do not separate accounting from tax, because on the ground they are not separate. We keep your books in the Odoo we configure for the PCN 2020 chart of accounts and Luxembourg tax rules, which means the result feeding your return is available continuously, not dug out under pressure at year-end. When the campaign comes, the return is prepared on an already-clean basis, filed on MyGuichet, and you know what you will owe early enough to plan for it in cash flow.

All of this sits inside the fixed monthly fee, from 325 € per month, all included: day-to-day bookkeeping, VAT and eCDF filing, annual accounts and register filing, tax returns, payroll, a monthly review with a dedicated manager, and real-time access to your books. No "return preparation" line landing as an extra in spring. What this fee covers, and what the market usually bills separately, is set out in our article on the cost of an accounting firm in Luxembourg.

Who this model is not for

Let's be straight. Our service is built for a Luxembourg SME of 1 to 50 employees that wants tax handled and anticipated, not just a return filed at the last minute. If you are a complex multi-jurisdiction holding with transfer-pricing issues, or your structure needs highly specialised, bespoke tax expertise, your need goes beyond a fixed-fee bookkeeping engagement. We will tell you at the first meeting rather than let you find out along the way. Our role is to inform and to keep your obligations current, not to stand in for specialist tax advice when the situation calls for it.

Frequently asked questions

Which form is used for a company's tax return in Luxembourg?

It is form 500, the return for corporate income tax and municipal business tax of undertakings. A single form is the basis for CIT, MBT and net wealth tax. It is filed electronically on MyGuichet.lu for resident capital companies.

Is online filing mandatory?

Yes. Since tax year 2017, resident commercial undertakings (Sàrl, Sàrl-S, SA, SAS, SCA, SE) must file their return electronically through MyGuichet.lu, using a LuxTrust product. Paper filing is no longer available for these companies.

What is the filing deadline in 2026?

For tax year 2025, the forms have been available since 7 April 2026 and the submission window runs until 31 December 2026, according to the Luxembourg Inland Revenue calendar. Exact dates should be confirmed each year on the official tax calendar.

What is the difference between MyGuichet and eCDF?

MyGuichet.lu is used to file the annual tax return (form 500). The eCDF platform is used for VAT returns and for filing the financial data from annual accounts. These are two separate obligations on two separate channels.

What is the risk of filing late?

A late or missing return exposes the company to an estimated assessment set by the authority, generally unfavourable, plus possible surcharges. It is better to file once the accounts are closed, without waiting for the final deadline.

Further reading

• Corporate tax in Luxembourg: CIT, MBT and net wealth tax explained
• Filing annual accounts in Luxembourg: deadlines, late fees and preparation
• Luxembourg VAT returns: filing frequency, eCDF and deadlines
• Accounting firm in Luxembourg: the complete guide for an SME
• What an accounting firm costs in Luxembourg: fixed fee, hourly rates and hidden extras

Why Advena?

We are the only Luxembourg accounting firm that keeps your books in the management tool it deployed for you. The result: the profit feeding your return is up to date all year, the return goes out on time through MyGuichet, and you know your tax early enough to plan for it. A fee set in advance from 325 € per month, a named manager, and not one surprise invoice line at tax-campaign time. We inform on the rules and keep your obligations current; we invent nothing and point you to specialist advice when your situation calls for it.