Lowerings linked to your adult children

Children who turned 18 

If your child reached age 18 in 2025, there are several options for how they should be treated for income tax purposes.

1. You declare him or her as a minor, attached to your household. In this case, you benefit from the usual increase to the family quotient and declare his or her income up until the age of 18. For the part of the year after that, your child can make their own declaration.

2. You do not declare the child as being attached and thus do not get their extra part/half part. You do not include their income in your declaration and instead can make a deduction for support money you pay them after age 18. Your child must make their own declaration for the whole of 2025’s income, including your support money (but only up to the limits of what is deductible from your income tax – see chapter 7, 'Lowerings linked to your adult children').

3. Your child asks to be ‘attached’ after age 18. They have no personal declaration to make; you declare any income they may have; you benefit from their extra part/half part.

Which adult children can be ‘attached’ to your household?

Children aged 18 or over on January 1 of the year for which you are declaring (so 2025 in 2026) usually have to fill out their own declaration but there are exceptions.

Single adults aged under 21 – or under 25 if in full-time study – can request that you attach them to your tax household (if their parents are separated the adult child can only be attached to the tax household of one parent). 

There can be pros and cons. 

For example, if you fall into the 30% tax band or higher, it may be more beneficial to stay separate but give support money that you can deduct, since the ceiling on the benefit of the family quotient will probably remove much advantage from an extra part or parts from attaching your child. 

However, attaching a child gives rise to a tax reduction for educational expenses and also increases the maximum income you can have while benefiting from advantages such as a means-based reduction in the taxe foncière property tax.

Children who live abroad and meet the criteria (eg. because they are studying in a foreign university) may be attached to your household as long as there is nothing in the double tax treaty with the country concerned that bans this. 

There is nothing to ban this for example with UK and US agreements and children studying there can be attached. If you do opt for this, each year, your child should write and sign a note requesting to be attached, that you should retain for your records.

As with any child forming part of your tax household, an attached adult child gives an extra part or half part depending on how many other children you have. See the tax band sections in chapter 5).

Disabled adult children

If you have children who are infirm and cannot manage independently due to disabilities, they are considered attached to your tax household whatever their age, unless they make their own declaration. This can apply even if they do not live with you.

In this case, the disabled adult child gives rise to an additional family quotient part under the same rules as minor children, unless he or she has a disabled person’s card showing 80% or more disability, in which case there is an extra half part. 

A disabled adult child can also opt to make their own declaration if they wish, in which case you can treat support money paid to them as a deductible expense (see below). 

Married/pacsed children

In general, adult children who are married or pacsés will form their own foyer fiscal.

However, members of a young couple may ask that they and any of their own children be ‘attached’ to the household of either of their parents if the child or their partner meets the criteria mentioned above for adult children (this applies whether or not they live under their roof).

In this case, there are no additional parts, but you benefit from a tax allowance that you can deduct from your declarable income, of €6,855 per person attached in this way. 

Your child should write a signed, dated request to be attached to your foyer fiscal for the relevant tax year, which you should keep in your records. The above also applies to children who are single parents.

Adult children in need

If one of your adult children lives with you but is not attached to your tax household you can opt to claim a fixed deduction sum of €4,075 for their board and lodging if they are unable to support themselves. 

The amount is pro rata if they only lived with you for part of 2025.

If the fixed amount is not sufficient you can deduct ‘real’ expenses up to €6,855 but the tax office may request proof of these. 

You can also retain the fixed amount for their board and lodging and add a further sum within the higher ceiling for other essential costs such as health or schooling.

If your child in need does not live with you then just the ‘real’ expenses rule and the €6,855 ceiling applies.

If your adult child is married or pacsed the ceilings can be doubled if the parent of the spouse/partner is not providing similar help. In the case of adult children who are single parents, the amounts can be doubled.

A child over 18 whom you support must also declare the same figure on their declaration, on the 2042, box 1AO/1BO (1CO/1DO if a dependent of someone else’s household).

These deductions relate to money you pay because your child needs it to get by.

In one case, a court ruled that money paid to a woman by her parents was not allowed for this as she had not made any attempt to apply for money she was owed from her ex-husband or welfare benefits she was entitled to.