Filing Your Box 3 Wealth Tax Return: A Step-by-Step Walkthrough

Our Box 3 guide covers how the deemed-return mechanics work. This one covers the part that actually catches expats out: the Belastingdienst portal pre-fills your Dutch bank and investment balances for you, which quietly trains people to assume the form is complete when they haven't touched it — and then it doesn't pre-fill anything held outside the Netherlands at all.

Before you log in: gather your January 1st numbers

Everything in Box 3 is assessed as of January 1st of the tax year, so gather these before you start rather than mid-form:

Every Dutch bank and savings account balance as of January 1st (the portal will likely already have this, but check it against your own statement).
Every foreign bank account balance as of the same date, converted to euros — these will not be pre-filled.
Your broker's January 1st portfolio statement, Dutch or foreign. Our broker comparison guide covers why how easy this export is varies a lot by platform.
Debts that reduce your Box 3 position (excluding a mortgage on your primary residence, which lives in Box 1).
Your BSN and DigiD.

Logging in and finding the Box 3 section

Go to mijn.belastingdienst.nl and log in with DigiD.
Open Aangifte inkomstenbelasting for the relevant tax year — the filing window for a given year's return typically opens March 1st the following year.
Work through to the Box 3: Belastbaar inkomen uit sparen en beleggen (savings and investments) section. It's broken into categories — bank and savings balances, investments, other assets, and debts — rather than one single number.

What's pre-filled, and what genuinely isn't

This is the part worth reading twice. The Belastingdienst receives automatic reporting from Dutch banks, brokers, and insurers, so your Dutch account balances typically show up already populated when you reach this section — the vooraf ingevulde aangifte (pre-filled return). It's easy to see numbers already sitting there and assume the whole section is handled.

Foreign accounts are not part of that automatic reporting. A savings account back home, a brokerage account with a non-Dutch platform, foreign property, foreign crypto holdings — none of it arrives pre-filled, because the foreign institution isn't required to report to the Belastingdienst the way a Dutch one is. You have to add every foreign asset yourself, under the relevant "other assets" or foreign-assets entry in the same Box 3 section. As a Dutch tax resident, you owe tax on worldwide wealth — the portal simply doesn't go looking for the foreign half of it on your behalf.

Deadlines, extensions, and what happens if you owe

The standard deadline to file is May 1st of the year after the tax year in question.
If you need more time, request an extension (uitstel) inside Mijn Belastingdienst before May 1st — approved extensions typically run to September 1st, and you don't need to give a reason if you ask before the deadline. After May 1st, an extension request needs a reason and isn't guaranteed.
An extension doesn't pause tax interest. If you end up owing money, interest generally starts accruing from July 1st regardless of whether you filed on time or extended — filing early if you expect to owe is the only way to avoid paying for the delay.

If you moved into or out of the Netherlands during the year

Don't use the standard annual form. If you became or stopped being a Dutch tax resident partway through the tax year, you file an M-form instead, which prorates your Box 3 position to the months you were actually resident — see our leaving-the-Netherlands exit checklist for what else that transition year involves.

Do this every year, not just the first time

The pre-fill convenience is exactly why this is worth re-checking annually rather than assuming last year's habits still cover you: a new foreign account you opened this year, a brokerage you switched to, a property you bought abroad — none of it will announce itself in the portal. The Box 3 numbers change every January 1st, and so does what you personally need to go find and enter by hand. If you're building toward financial independence, see why this drag matters for your FI number.