27. Forms

In Luxembourg, income tax returns are based on different forms, each designed for a specific tax situation. The choice of form depends in particular on your residency status, the nature of your income, and your personal or family situation.

It is therefore essential to correctly identify the form corresponding to your case in order to avoid errors or delays in processing your file.

Which form is used for which purpose?

Main tax return forms

Forms 100 (tax return) and 163 (annual statement) are the main forms used to report your income and expenses.

Depending on your personal situation, you must submit either form 100 or form 163, but never both.

Other tax return forms (non-exhaustive list)

The following forms may be required to report certain types of income in more detail than allowed by forms 100 or 163, or for specific administrative reasons.

Form 101 – Power of attorney

If you submit a joint tax return with your partner via your MyGuichet.lu account, it may be necessary to attach Form 101, which confirms that you are authorized to represent your partner.
In this case, your LuxTrust signature on MyGuichet.lu is sufficient to sign the joint declaration for both spouses.
Form 101 is also required if you submit a tax return on behalf of a third party (e.g., your parents) from your own MyGuichet.lu account.

Form 200 – Declaration for the joint determination of income from collective businesses and co-ownership

This form is used to declare profits from co-owned properties or jointly conducted activities.
It determines the allocation of total profit among co-owners. Each taxpayer must then report their individual share in the main form 100. Form 200 is often required when declaring income from co-owned real estate to determine each co-owner’s share of rental income.

Forms 141 and 144 – Agricultural and viticultural activities

These forms are used to declare professional activities as a winemaker (Winzer) or in the agricultural sector. They are generally required when these activities represent your main profession.

Form 195 – Income from leasing or renting undeveloped properties

This form is used to declare income from:

• leasing or renting land (Pacht) and
• other agricultural income (e.g., sale of timber).

It is generally required for taxpayers who receive occasional agricultural or forestry income as part of a secondary activity (agricultural and forestry profit).

Form 152 – Business expenses (liberal professions)

This form is used to declare all expenses related to self-employed or freelance activities.

Copies of all invoices must be attached to this form when submitted to the tax administration.

Form 180 – Annex: Net income from movable capital

This annex allows you to list your income from movable capital (e.g., interest, dividends, capital gains).
It is generally not required, except on explicit request of the tax administration. In most cases, an annual financial statement is sufficient unless otherwise instructed by the authorities.

Form 190 / 210 – Income from renting built properties

For tax years 2023 and earlier, forms 190 and 210 were separate.
From tax year 2024, they were merged into a single form: 190/210.

This form is used to declare rental income from properties in Luxembourg or abroad.
All information regarding rented properties is reported in detail to determine net rental income, which must then be reported in form 100.

Form 700 – Income from capital gains on private real estate

Form 700 is used to declare capital gains from the sale of real estate located in Luxembourg.

Depending on your situation, the sale may be tax-exempt or taxable.

When should forms be completed?

Tax return forms for tax year N must be submitted no later than December 31 of year N+1.

Example: The 2025 tax return must be submitted by December 31, 2026.

Note: simply dropping the documents in a mailbox on December 31 may result in late submission. Documents must reach the tax administration no later than December 31.

Do I need to complete all these forms?

No. You only need to complete the forms necessary to declare your income and expenses for the relevant tax year.
Form 100 must be submitted if you meet the conditions that require you to file a tax return for that year.

All other forms must only be submitted if:

• you receive income that must be declared using these forms, or
• the tax administration expressly requests it.

Forms 166 et 164

The tax withholding certificate is a fiscal document issued by the Administration des contributions directes (ACD) that allows your employer or pension fund to calculate and withhold tax at source on your salary or pension in Luxembourg.

Certain personal events or tax choices require completing specific forms to modify this certificate or your withholding rate.

The two most common forms in this context are:

• Form 166: to modify your withholding tax method (tax class, rate, etc.).
• Form 164: to notify a change in personal situation or request the creation/modification of a tax withholding certificate.

Form 166: Request for modification of taxation at source

What is form 166 ?

Form 166 is a tax document used to request a change to the withholding tax method recorded on your tax withholding certificate.

This concerns, in particular:
switching from joint taxation to individual taxation (tax class 1),

• choosing individual taxation with income reallocation,
• requesting a fixed rate (e.g., for married cross-border couples),
• revoking or adjusting an already existing withholding method.

In practice, form 166 is useful if you want your withholding rate to more accurately reflect your family situation or income.

Who can complete Form 166?

This form can be completed by:

• residents employed or receiving a pension in Luxembourg,
• cross-border workers employed or receiving a pension in Luxembourg,

when they want to modify their withholding tax method, for example:

• a married cross-border couple who wants individual taxation or a fixed rate,
• a couple who wants to return to a standard taxation regime,
• a resident couple who wants joint taxation (class 2).

When should Form 166 be completed?

You can fill out this form at any time during the year. Once completed, signed, and sent to your RTS office (withholding at source), you will receive an updated tax withholding certificate within about one month.

What documents are required to complete Form 166?

You will need to provide a copy of your last three pay slips as well as an estimate of any other income (outside salaries), if applicable.

Unable to complete form 166 yourself? Don’t worry, taxx.lu can do it for you for €59!

Form 164: Change in marital status and amendments to the tax return

What is form 164?

Form 164 is used to create, correct, or update a tax withholding certificate, particularly when your personal situation or data changes.

This is especially relevant in the following cases:

• change of marital status (marriage, divorce, separation, widowhood),
• change of personal address,
• request for a new tax certificate,
• request to correct an existing certificate.

For residents, many changes are normally handled automatically by the ACD as soon as personal data is updated in the National Register of Natural Persons (RNPP). For non-residents, it is often necessary to submit Form 164 NR to notify these changes.

Who can complete Form 164?

Form 164 should be completed when:

• you have not received your tax withholding certificate,
• you notice an error on your tax withholding certificate,
• your marital status changes (marriage, divorce, civil partnership…),
• you have moved abroad or within Luxembourg.

In the case of a change in marital status, this form allows you to update your tax class or withholding rates without delay.