Short answer: if you rent out a home to a private tenant who does not use it for professional purposes, Belgium does not tax you on the rent you actually collect. You declare only the property's non-indexed cadastral income, and the tax office works out the taxable amount itself using the formula indexed cadastral income × 1.40. The rent figure never appears anywhere on your return.

This surprises almost every first-time landlord, and it is the single most common reason people over-declare and pay more than they owe. Below is exactly what goes on the return for income year 2025 (tax year 2026), with the official figures, worked examples, and the three situations where the rule changes completely.

Region flag: this is federal tax and works identically in Flanders, Brussels and Wallonia. Do not confuse it with the regional property tax (précompte immobilier / onroerende voorheffing), which is a separate annual bill — that one is covered in our guide on whether a landlord can charge property tax to a tenant. Your own home is exempt and is not what this article is about.

What do you actually put on the return?

For a built property let to a private individual who does not use it professionally, you enter one number in box III (Income from immovable property):

• Code 1106 / 2106 — the non-indexed cadastral income of the property.

That is it. No rent, no service charges, no deposit. You can find the non-indexed cadastral income in MyMinfin (My home > Consult my immovable property data), in the online return wizard, or on your property-tax assessment.

How is the taxable amount calculated?

You do not calculate it — the tax office does, automatically, from the number you declared. The formula is:

Taxable income = (non-indexed cadastral income × indexation coefficient, rounded to the nearest euro) × 1.40

Income year	Tax year	Indexation coefficient
2025	2026	2.2446
2026	2027	2.3000

Worked example. Your flat has a non-indexed cadastral income of €450 and you let it to a retired couple.

1. Declare €450 next to code 1106.
2. Indexed cadastral income: 450 × 2.2446 = €1,010 (rounded).
3. Taxable immovable income: 1,010 × 1.40 = €1,414.

So a flat generating, say, €9,600 of rent a year adds €1,414 to your taxable income — not €9,600. The result appears on your assessment notice under Detail of the calculation > Income from immovable property, and it is taxed at the progressive rates on top of your other income.

What if the tenant uses the property for their business?

This is where the rule flips, and it is expensive. If you let to a natural person who uses the property professionally, or to a company, association or other legal entity, you are taxed on the actual rent. You declare two numbers:

• Code 1109 / 2109 — the non-indexed cadastral income.
• Code 1110 / 2110 — the gross rent, meaning the rent plus any rental advantages (expenses the tenant pays in your place, such as taxes, major repairs or insurance premiums).

The taxable amount is then the higher of two figures: the net rent (gross rent minus a flat 40% for costs) or indexed cadastral income × 1.40. Crucially, that 40% deduction is capped at two-thirds × non-indexed cadastral income × the revalorisation coefficient, which is 5.63 for income year 2025.

Worked example. Non-indexed cadastral income €1,000; gross rent €14,400 for the year; tenant is a self-employed consultant using the property as an office.

• Flat 40% of €14,400 = €5,760, but the cap is 2/3 × 1,000 × 5.63 = €3,753.33, so the deduction is limited to €3,753.33.
• Net rent = 14,400 − 3,753.33 = €10,646.67.
• Alternative figure = (1,000 × 2.2446 = 2,245) × 1.40 = €3,143.
• Taxable = the higher of the two = €10,646.67.

Roughly three and a half times the tax base of the same property let to a private tenant. This is why the professional-use clause in a lease matters far more than most landlords realise — and why you want to know, in writing, what your tenant will use the property for before you sign.

What about furnished letting?

Furnished letting is split into two income streams. The property side stays in box III under code 1106/2106 as usual. The furniture side becomes movable income in box VII:

• Code 1156 / 2156 — the net rental income from the furniture, taxed in principle at 30%.
• Unless your lease splits the rent explicitly, the furniture share is set at a flat 40% of the total rent, and costs are estimated at a flat 50% of that.

On €12,000 of total annual rent: furniture gross = €4,800, costs = €2,400, so €2,400 goes in code 1156 and is taxed at 30% — about €720. If you also charge for services such as cleaning or breakfast, that fee is declared separately as miscellaneous income in part 2 of the return.

What if I bought, sold or co-own the property?

You declare only your share, and only for the days you held it.

• Bought or sold mid-year: pro-rate by days. A property with a cadastral income of €1,200 bought on 15 March 2025 gives 1,200 × 291/365 = €956.71 to declare.
• Co-ownership: declare your ownership share. Two sisters owning 50/50 a property with a cadastral income of €1,000 each declare €500.
• Married under the legal (community) regime: immovable income is always common, even if only one spouse owns the property — so each declares 50%.
• Married under separation of property, or legally cohabiting: each partner declares according to their own ownership share.

The mistakes that cost landlords money

• Declaring the rent instead of the cadastral income. The most expensive error there is, and the easiest to make if you are used to another country's system.
• Using the indexed figure in code 1106. The box wants the non-indexed cadastral income; the indexation is applied for you.
• Forgetting the pro-rata after a purchase, sale or inheritance.
• Missing a change of use. A tenant who registers their business at your address moves you into the professional regime — and you are the one assessed.
• No clean record of what was received. If the tax office queries a professional letting, you need the year's rent and rental advantages documented, not reconstructed from bank statements in June.

That last point is where most of the pain actually sits. Rent received, indexations applied, charges settled and property tax paid live in four different places for most landlords, and a return that takes ten minutes with clean records takes a weekend without them. If you keep your leases, rent ledger and documents in one place — as ImmoDesk does with rental accounting and automatic document filing — the numbers for box III are simply there when you need them. And when the annual indexation comes around, our rent indexation calculator gives you the correct figure in seconds.

Before you file

Get three things straight: the non-indexed cadastral income of each property, exactly how each tenant uses it, and the days and share you owned it. With those, box III takes minutes. Filing deadlines differ between paper and online returns and are published each year by the FPS Finance — check them in MyMinfin rather than relying on last year's date.

This article is general information for landlords in Belgium and is not legal or tax advice. Rules change and individual situations vary; for your own return, check the official pages of the FPS Finance (fin.belgium.be) or speak to an accountant.

Spend less of your year chasing your own numbers. ImmoDesk keeps leases, rent, charges, indexations and documents for every property in one place, so tax season is a lookup rather than a reconstruction. Start your 7-day free trial — no credit card required.