Is registered mail required to notify an indexation?

Brief answer

In Belgium, registered mail is not legally mandatory to notify a rent indexation. Article 1728^bis of the Civil Code only requires a written document. A simple letter, an explicit email or a qualified electronic registered mail (eIDAS Regulation) are legally valid. In practice, registered mail remains strongly recommended to have formal proof of the date of the request — which fixes the 3-month retroactivity window of the request (art. 1728bis, §1 of the Civil Code).

What the law exactly says

Article 1728^bis §2 of the Belgian Civil Code formulates the requirement as follows:

« § 2. The indexation will only be applied after a written request made by the interested party. (…) »

Three keywords:

• « written » — no oral mention, no implicit request
• « request » — no informational mention, an explicit indexation request is required
• « by the interested party » — the landlord themselves or their representative (agency, syndic)

No mention of a specific format: no registered mail required, no AR required, no prior notice required. The law is deliberately flexible on form and strict on substance (lease registration, mathematical formula).

Comparative tableau of possible forms

Form	Legal validity	Cost	Proof of receipt	Recommended in practice?
Simple letter posted	Yes	~1.80 €	None	For trusting relationships
Simple letter handed in person	Yes	0 €	If signed by the tenant	Risky if tenant doesn’t sign
Explicit email	Yes (Code economic law XII.16)	0 €	Weak (spam, wrong email)	For daily traceability, not alone
Paper registered mail without AR	Yes	~4.50 €	Date of presentation	Acceptable compromise
Paper registered mail with AR	Yes	~7.50-8 €	Firm date of receipt	Recommended standard
Qualified electronic registered mail	Yes (eIDAS Regulation)	Varies by provider	Timestamped proof of dispatch and delivery	Via an accredited provider
Bailiff (formal service)	Yes	~100-150 €	Date of instrumentation	Severe conflict cases only

For 90% of cases, paper registered mail with AR is the optimal format: 7-8 € for indisputable evidence.

Why registered mail is strongly recommended in practice

Reason 1: proving the date of the request

Indexation applies at the earliest on the anniversary of the lease’s entry into force, once requested in writing — it does not take effect on receipt. But the date of the request matters: it fixes the 3-month retroactivity window of a late request and serves as evidence. This date determines:

• The 3 months preceding the month of the request that a late request can reach back over (art. 1728bis, §1)
• The calculation of any arrears in case of late notification

Without formal proof of this date, it is one’s word against the other’s. The landlord claims having sent in April, the tenant claims having received in July — the judge will decide based on the elements (and often in the tenant’s favour in case of doubt).

Reason 2: the cost is negligible compared to the stakes

For an indexed rent increasing by 30 €/month:

• Annual cost of a registered AR mail: ~8 €
• Cost of a judicial dispute: court fees + time
• Indexed rent at stake over 1 year: 360 €
• Cumulative indexed rent over 5 years: 1,800 €

The benefit/cost ratio is 1:45 at one year and 1:225 at five years. No economic reason to do without.

Reason 3: psychological effect on the tenant

A registered mail is perceived as a serious approach. It signals to the tenant that:

• The landlord takes the procedure seriously
• The request is legally prepared
• Any contestation will itself have to be formalised

This « solemn » effect reduces frivolous or pure-form contestations — a tenant who receives a registered mail will hesitate to ignore it.

Reason 4: coverage in case of changing behaviour

A tenant on good terms today may become conflictual tomorrow (change of personal situation, conflict on another subject, end of lease). The registered mail kept constitutes a reserve of proof mobilisable several years later.

Electronic registered mail: a qualified alternative

Qualified electronic registered mail exists under the European eIDAS Regulation: sent through an accredited provider, it has the same legal effect as paper registered mail (proof of dispatch, of delivery and of their date). Two points to know:

• bpost does not offer electronic registered mail. My eBox is not a service for sending documents between private individuals either — it only receives documents from public institutions
• Choose a provider accredited for qualified electronic registered delivery, and keep the timestamped certificate of dispatch and delivery in your tenant file

This format suits landlords managing multiple properties with digital tools. See our registered mail letter template for practical detail.

Practical cases

Case 1: First indexation, recent tenant

Situation: lease signed 1 year ago, first indexation to notify. Recommendation: registered AR mail (paper or qualified electronic). Educational effect, clear fixation of the legal framework, no trust history yet established.

Case 2: 5th indexation, loyal tenant of 5 years

Situation: excellent relationship, punctual payments, open communication. Recommendation: simple letter or explicit email. The risk of dispute is marginal, the saving of 8 €/year over 4 years (32 €) remains symbolic but legitimate.

Case 3: Tenant 3 months late on payment

Situation: established tensions, erratic payments. Recommendation: paper registered AR mail strongly advised. Combine with a formal notice for arrears. Prepare the file for the justice of the peace.

Case 4: Lease signed under agency management

Situation: the agency manages 50 properties and notifies automatically. Recommendation: systematic registered mail, paper or qualified electronic. Centralised, timestamped, archived. Integration with the agency’s professional software.

Case 5: Tenant unreachable by post (move)

Situation: mail returns regularly « unknown at this address ». Recommendation: bailiff service (~100-150 €). It is more expensive but guarantees personal delivery even if the tenant avoids. Indispensable if the financial stake exceeds 500 €.

What does Belgian case law say?

Several decisions of Belgian justices of the peace have confirmed the legal validity of:

• The simple letter as an indexation act, on condition that it has been received (proof to be borne by the landlord)
• The email as a valid written act, subject to the tenant not contesting having received it
• The tenant’s refusal to sign the AR does not prevent the effect of notification (the postman’s « presented » mention counts as proof of attempted delivery)

Conversely, the more informal the medium, the weaker the evidence:

• SMS or WhatsApp message: a written request in principle — Wallonia expressly admits « ordinary letter, SMS or e-mail », provided proof is kept — but the date and the content are harder to establish in case of dispute; the risk is evidentiary, not one of validity
• Post-it notes slipped under a door (no proof of voluntary delivery)
• Oral mention during a visit (no written request at all, so no indexation)

Conclusion: « written » yes, but keep proof of the date and the content. Registered mail is the format that eliminates any ambiguity.

To go further

• Registered mail template for indexation — ready-to-print PDF template
• How to notify the tenant — mandatory letter elements
• When to notify the tenant? — optimal timing
• Indexation notification template — complete form template
• Tenant refusal: remedies — escalation in case of refusal
• Contesting an indexation — tenant perspective
• Calculate the indexation — verify the calculation before sending
• Methodology — legal sources

Registered mail is not legally mandatory. It is in good practice. For 7-8 € per year, you buy the tranquillity of indisputable proof — the best value-for-money in Belgian rental management.