Meerwaardebelasting: the opt-out choice, the 31 August ceiling, and the bank cut-offs that came first

The statute gives your client until 31 August 2026 to make the choice known. The institution may not. Both are true at once, and the distance between them is this week's work.

The levy on realised capital gains on financial assets took effect on 1 January 2026 under the Law of 6 April 2026 (Belgian Official Gazette, 21 April 2026, numac 2026002780). Financial institutions began withholding on 1 June 2026. Between those two dates sits a transitional architecture that most zaakvoerder clients with a personal securities account have not read, and that you now have about a week to close out per file.

What the choice actually decides

Art. 265/1 WIB 92 lets the taxpayer settle the levy in one of two ways.

• Withholding retained. The institution withholds at source. The withholding is bevrijdend — it discharges the liability, and the gains do not have to be carried into the personal return.
• The opt-out. The institution does not withhold. The realised gains go into the personal income tax return, where the taxpayer computes and declares the position himself.

Three properties of that choice get misread, and each one changes a file:

1. It is not a June-to-August choice. The choice made by 31 August 2026 governs income year 2026 as a whole, not only the gains realised in the window during which it happened to be communicated.
2. It runs on until revoked. This is not an annual election that lapses. Whatever is on file with the institution keeps applying to later income years until the client changes it.
3. Late is not late for 2026 — it is early for 2027. Art. 34 is explicit: a choice communicated after the deadline takes effect only from the following taxable period. A late opt-out does not reopen income year 2026.

Two deadlines, and one of them has already passed for most clients

Reported institution cut-offs, all of them before 31 August:

Treat that table as a prompt, not as authority. The only date that binds the counter is the one the institution itself published, and it is worth reading the notice rather than a summary of it — some houses distinguish between account types, between execution-only and discretionary mandates, and between an instruction given in the app and one given on paper.

The Council of State says the decree cannot do that

The Council of State, in its opinion on the Royal Decree of 18 May 2026, took the position that art. 85/1 contradicts the enabling statute: the taxpayer must be able to make the choice known up to and including 31 August 2026, and a decree cannot shorten a period the law grants. That advice did not stop the decree.

So the honest description for a client who is past his bank's cut-off but inside the statutory period is: you have an argument, not a right anyone can enforce for you before 31 August. Do not promise that a particular institution will accept a late instruction. What you can do is make the argument exist on paper:

• Send the instruction in writing, dated, through a channel that produces a record — secure message in the bank environment, or registered letter for the larger positions.
• Keep the refusal if one comes back, in the words the institution used.
• Note in the client file which route the client is actually on as at 31 August 2026, and on what evidence.

If the conflict is ever litigated, the client who wrote in time and was refused is in a different position from the client who wrote nothing.

Gains realised between 1 January and 31 May 2026

For that first stretch of the year no withholding machinery existed. Art. 35 provides a repair: the withholding agent may, on the written request of all account holders, pay over an amount equivalent to the roerende voorheffing on those gains. The remittance is due at the latest on 30 November 2026, and the amount paid over is treated as tax actually withheld for the purposes of arts. 307 and 313 WIB 92 — which is what removes the declaration obligation.

Two words in that sentence do the work. May: the institution is permitted, not obliged, so a request can simply be declined. All account holders: a joint account, a split between spouses, a usufruct arrangement or an indivision needs every holder's signature. The law conditions the request on all of them and does not say what becomes of an incomplete one, so do not assume a partial set is partially effective. Check the account-holder structure before drafting, not after.

Where the facility is not used or not granted, the 1 January - 31 May gains belong in the return, regardless of what was chosen for the rest of the year.

The insurance products where the default is reversed

Art. 34 also reverses the default for products falling under art. 92, par. 1, b) WIB 92 that were paid out between 1 June and 31 August 2026: the taxpayer is deemed to have opted out unless he explicitly asked for withholding.

This is the item most likely to be missed, because it produces an obligation out of silence. A client who received such a payout this summer and did nothing has a declaration to make. Pull the payout dates for the window and check them individually; do not infer the treatment from what the client chose on his securities account, because that choice does not govern here.

Clients with no Belgian withholding agent

The opt-out only exists where there is a Belgian withholding agent to communicate it to. A client trading through a foreign broker has nothing to communicate, no cut-off to check, and a declaration to make in any event. Separate those files out first — the deadline pressure does not apply to them, but the substantive work does, and it is larger.

The 31 December 2025 valuation is the base

The acquisition base for gains realised from 2026 onwards is the 31 December 2025 valuation — the fotomoment. Gains accrued before 2026 are outside the levy. That is the single most valuable document in the file and the one clients most often do not have.

Request it now, per position, in writing, from every institution the client holds an account with. It does not get easier to obtain years later when the return is questioned, and a valuation reconstructed after the fact from public price data is a weaker document than a statement the bank issued.

Aanmerkelijk belang, and why it is not this week

The carve-out for a direct holding of 20% or more works differently: a first tranche of EUR 1 million free per five-year period, then a band running from 1.25% to 10%. Its valuation window runs to 31 December 2027, so the file has a year and a half. It does not compete with the 31 August work, and mixing the two in the same client conversation is how the portfolio deadline gets missed.

The work order

1. List the clients with a personal securities account. Zaakvoerder files first — they are the ones where a portfolio sits beside a company that already occupies your attention.
2. Pull the 2026 realised-gain list per client, per institution, split at 1 June.
3. Pull the 31 December 2025 valuations per position, in writing.
4. Look up each institution's own published cut-off and record it in the file with the date you checked.
5. Confirm in writing with each client which route he is on, and get the instruction sent through a channel that leaves a record.
6. Check the 1 June - 31 August window for art. 92, par. 1, b) payouts, where silence means opted out.
7. Separate the foreign-broker clients — no communication to make, a return to prepare.

Everything that can still be influenced before 31 August is in steps 4 and 5. Everything else is preparation for a return that is being written either way.