Crypto assets and your tax return: substantiating origin for the tax office

The case: an entrepreneur with crypto profit and a question from the tax office

Declaring crypto in Box 3 only becomes difficult for many people once the tax office asks for substantiation. Suppose you are an entrepreneur and have built up crypto privately over the past years. In 2025 you took a nice profit. In the return for that year you declare your crypto holdings in Box 3, at the value on the reference date of 1 January.

A few months later a letter arrives: the Tax Authority wants to know where those assets come from and asks for substantiation of the value on that reference date. You have loose exports, a few screenshots and a wallet you manage yourself, but no overview that makes the whole picture watertight. The question of how to substantiate crypto in Box 3 then quickly lands on the table.

This situation is recognisable for many entrepreneurs and private individuals. The crypto is spread across an exchange, your own wallet and perhaps a hardware wallet. The tax office’s question is not whether you are acting in bad faith, but whether you can substantiate what you have declared. We frame the rest of this blog around that case, because it shows precisely what substantiation per reference date is all about.

In short: substantiating crypto for your tax return

• Substantiation revolves around the value on the reference date of 1 January and around the origin of assets: where does the crypto come from and how was it built up.
• For each reference date you record which holdings you had, on which addresses and exchanges, and at which rate, with sources that are verifiable.
• Substantiating is something other than advising: Paucitas maps out, verifies and tests independently; the fiscal interpretation you leave to your accountant or tax adviser.
• A clear crypto overview makes your bookkeeper’s work easier and gives the tax office a traceable picture instead of loose fragments.

How do I declare crypto in Box 3?

For private individuals in the Netherlands, cryptocurrency generally falls under Box 3, as other assets. That means it is not your profit during the year that counts, but the value of your holdings on the reference date of 1 January. So you declare what your crypto was worth at that single moment. The fiscal rules and rates around Box 3 change regularly and depend on your personal situation.

Exactly how you complete the return and which regime applies to you is a fiscal question. For that we refer you to your own accountant or tax adviser. What we do is deliver the factual substantiation on which that return can rest. How to declare crypto in Box 3 is also explained in the general guidance on saving and investing from the Dutch Tax Authority.

Recording the value per reference date for crypto: what do you need?

The core of substantiation is the value per reference date, measured on 1 January. To record that value in a verifiable way, you bring together a number of items. With crypto in Box 3 it is precisely that value per reference date that is the anchor point of your return.

Which data is needed?

• An overview of all the places where you hold crypto: exchanges, your own wallets and hardware wallets, with the associated addresses.
• The balances per wallet and per coin on the reference date, substantiated with the transaction history up to that moment.
• The rate per coin on 1 January, stating the source you use, so that the valuation is traceable.
• Documentation of the origin: purchase receipts, deposits from your bank account, or the moments at which holdings arose.

In the entrepreneur’s case this means we reconstruct the balances on 1 January from the transaction history, substantiate the rate on that date with a verifiable source and map out the route of the holdings up to the point at which they were purchased or received. This produces a value that does not rest on an estimate, but on recorded data.

The tax office asks about the origin of my crypto: what then?

When the tax office asks about the origin of assets, they want to be able to follow how your crypto holdings were built up. That is the same asset verification that also plays out at a bank or mortgage lender: not a judgement, but a traceable story of where the assets come from. For crypto this means we trace the chain of transactions back, from your current holdings to the moments of purchase or receipt, and link this to verifiable sources such as bank deposits or exchange purchases. Anyone who declares crypto in Box 3 must be able to substantiate that origin on request.

The result is a substantiation of the origin of assets that the tax office can read without there being gaps in the story. In doing so we assign no fiscal judgement and do not determine what you owe. We record the facts and make them verifiable. What those facts mean fiscally is assessed by your tax adviser.

A crypto overview for your bookkeeper or accountant

Many entrepreneurs engage a bookkeeper or accountant for the return. That professional can only do their work well with a clear crypto overview. Loose exports from different exchanges, in different formats and currencies, cost time and leave room for errors. We deliver an overview that brings together the balances per reference date, the valuation and the origin in a traceable way, so that your accountant can proceed directly with the fiscal processing. A clear overview makes processing crypto in Box 3 easier for your bookkeeper.

In this way the division of roles stays clean: we substantiate and verify the facts, your accountant provides the fiscal interpretation. That separation is important, because substantiating and advising are two different things. If you confuse them, you run the risk that substantiation is presented as advice, or the other way around.

Substantiating versus advising: where is the line?

Substantiating is making facts verifiable: which holdings you had, what they were worth, and where they came from. Advising is something else: determining how you process those facts fiscally, which regime applies and what you owe. Paucitas does the former and deliberately not the latter. We map out, verify and test independently, and deliver a report that serves as a factual basis. The fiscal and legal judgements we leave to the people trained for them: your tax adviser and, where necessary, your lawyer. We substantiate the facts around your crypto in Box 3; the fiscal interpretation we leave to your adviser.

Would you like to know how we approach substantiating crypto assets more broadly, separate from the return? Then read our blog on mapping out crypto assets. This blog deliberately focuses on building up the substantiation per reference date yourself; for the loose question-and-answer form around Box 3, a separate FAQ blog will appear later.

What if you have (partly) lost access to your own wallet?

Sometimes, while drawing up substantiation, it turns out that part of the assets sits on a wallet where access is no longer complete: a forgotten password or an incomplete backup. Provided the access codes are partly present, we investigate whether wallet recovery is among the possibilities, so that those holdings can also be included in the overview. That is only possible on a wallet of which you are the rightful owner yourself. Anyone searching for reliable crypto recovery would be wise to keep this distinction sharp: a reliable party never asks for your full seed phrase or private keys, and recovery is never guaranteed.

What substantiation by Paucitas delivers

As an expertise agency for blockchain investigation we deliver substantiation that stands. We reconstruct the balances per reference date, substantiate the valuation with verifiable sources and map out the origin of the assets, transaction by transaction. The result is a report that your accountant can use for the return and that the tax office can follow when substantiation is requested. We do this independently and without a fiscal or legal judgement: substantiating, verifying, mapping out and testing independently. This way, for crypto in Box 3 you have a file that makes the value and the origin traceable.

Calmly taking a first step

Are you in doubt whether your crypto records will withstand the tax office’s scrutiny, or is the Tax Authority already asking for substantiation per reference date? A short, no-obligation intake provides clarity. In it we discuss which data you have, what is still missing and what watertight substantiation could look like. After that you know where you stand, without committing to anything. The free intake is meant to think along with you, not to talk you into anything. Whether you are a private individual or an entrepreneur, substantiating crypto in Box 3 starts with a clear overview.