Underinsurance in car insurance: the value to declare

An insured value that is too low does not only reduce a total-loss payout. It trims every omnium claim in the same proportion, repairs included, once the gap is spotted. The proportional rule is defused before the accident, with an invoice, a purchase order and ten minutes of rereading.

What is the proportional rule in car insurance?

A mechanical cut in the payout. If you insured your car for less than the value the contract requires, the insurer only pays in the ratio between the declared value and the value that should have been declared. English-speaking readers may know it as the average clause.

The legal basis is Belgian and fits in one sentence. Article 98 of the Act of 4 April 2014 on insurance provides that, unless agreed otherwise, when the amount insured is lower than the determinable value of the insured interest, the insurer is only bound in that ratio. Car contracts repeat it almost word for word.

AXA's Confort Auto general conditions, issued under the L'Ardenne Prévoyante brand in June 2025, give the official example: declared value 10,000 euros, value that should have been declared 12,500 euros, loss 2,500 euros. The payout drops to 2,000 euros.

Two rules share the same name, and they are often confused.

This article deals with the first. The second is covered in our article on false declarations.

Which value should you declare for your omnium?

The one your general conditions define, and they do not all say the same thing.

The same insurer changed its base between two editions. A value that was right in 2019 may be wrong under the contract you signed last year.

One common thread runs through all these texts: the value is declared "under the responsibility of the policyholder". Neither the dealer, nor the bank, nor the broker signs it for you, even when they fill in the form.

Does the rule also apply to a simple repair?

Yes, and that is the trap.

People tend to assume that underinsurance only costs money after a theft or a write-off, since that is when the value serves as the basis of calculation. The texts say otherwise. Ethias applies the proportional rule "in the event of a claim", with no distinction. AXA provides for it expressly on a total loss and on a repair, and places it in its calculation before the excess is deducted. Every dented door, every torn-off mirror, every spring hailstorm therefore goes through the same coefficient until the value is corrected.

A worked example, fictitious but typical. Your car is invoiced at 32,000 euros including VAT, of which 6,000 euros is an options pack on a separate purchase order. You declare 26,000 euros, the "base" price. The ratio is 81.25%. A 4,000 euro bodywork repair becomes 3,250 euros, then 2,750 euros after a 500 euro excess. With the right value you would have received 3,500 euros.

750 euros lost on a single wing.

Options and accessories added after purchase: a ceiling of 1,000 to 1,240 euros

Above that ceiling, you must declare them.

At AXA, equipment fitted after purchase must be reported, and if it is not, it is covered up to 1,240 euros excluding VAT. The contract's own example: a radio installation worth 1,500 euros excluding VAT, not declared, is only insured up to 1,240 euros. The anti-theft system the insurer requires is always covered. The older Ethias edition insured accessories fitted after signing free of charge up to 1,000 euros excluding VAT, on a dated and paid invoice. It also excluded lettering from the definition of an accessory, and reminded readers that items carried in the car never form part of the insured value.

Before signing, check that your value includes:

• the options pack invoiced separately on the purchase order;
• metallic or pearl paint, often counted as an option;
• wheels, tow bar or roof bars fitted by the dealer before delivery;
• the mobile charging cable of an electric or hybrid car;
• the registration tax, if your contract lets you add it;
• any equipment fitted after purchase that exceeds your contract's ceiling.

A maintenance contract bundled into a "pack" has no place on this list: AXA excludes it from the insured value.

How do you declare the right insured value in six steps?

Allow twenty minutes, once, then again whenever you add equipment.

1. Open the general conditions and search for "valeur assurée" (insured value). Note the required base: catalogue or invoice, excluding or including VAT, discounts deducted or not.
2. Recalculate that value from your purchase documents, factory options included.
3. Compare it with the amount written in your specific conditions, to the euro.
4. Add up the equipment fitted after purchase and set the total against your contract's ceiling.
5. Ask for a written endorsement if there is a real gap, with supporting documents attached.
6. File the invoices as photos on your phone, next to your set of mandatory car documents.

After a claim, step 5 comes too late.

Value already too low: the endorsement before the claim

Correct it now. An endorsement raises the omnium premium, because the insured value is one of its rating criteria, but the increase is measured as a percentage of the premium whereas the proportional rule is measured as a percentage of every payout. The comparison settles quickly. Deliberate under-declaration, on the other hand, leaves the territory of the proportional rule and enters that of false declaration, where the penalty can go as far as voiding the contract. And declaring too high earns you nothing: the payout is in principle still calculated on the value defined by the contract and its schedule, explained in our article on market value, agreed value and new-for-old.

To compare omnium policies on their value base and their exclusions, the comparison tool lines up the cover side by side, the ranking of the best car insurance in Belgium details what each company covers and refuses, and the quiz helps you choose between mini-omnium and full omnium. If a claim is already under way, the steps are in our article on reporting a claim.

The Wikifin portal run by FSMA, the Belgian financial regulator, explains how omnium works, and FSMA supervises the conduct rules of insurers and intermediaries. In a dispute over a reduced payout, the Insurance Ombudsman, Belgium's free out-of-court complaints body for insurance, examines complaints at no cost. Insurer documents consulted on 21 October 2026: the AXA Omnium XL Pro general conditions, the Ethias omnium general conditions and the Beobank Car Insurance information sheet.

This site informs and compares. It is not an insurance intermediary and gives no individual recommendation.