Before you start

Steuer-ID, issued automatically after Anmeldung
Understanding of your tax class
A day-by-day record of where you worked, if you cross a border
Records of foreign income and foreign accounts
An ELSTER account, if filing yourself

Step-by-step

1
Give your employer your Steuer-ID as soon as it arrives
Until they have it, payroll applies the least favourable class and a large amount comes off your pay — recoverable at filing but painful in your first months.
Via employerWho: YouAs soon as issued
2
Understand your tax class
Steuerklasse determines monthly withholding: class 1 for single people, 3/5 or 4/4 combinations for married couples. Choosing 3/5 where incomes are uneven reduces monthly withholding, though the annual liability settles at filing either way.
OnlineWho: You
3
If you work across a border, keep a working-days calendar from day one
Employment income is taxed where the work is physically performed. A day worked at your Aachen kitchen table for a Dutch employer is German-taxed; a day in the Maastricht office is Dutch-taxed. At the end of the year somebody has to allocate the income, and reconstructing that from memory in April is where cross-border cases go wrong.
OnlineWho: YouFrom your first working day
4
Check your church tax position
If you declared church membership at Anmeldung, 9% of your income tax is deducted automatically in NRW. Leaving formally requires an appointment and a fee, and takes effect from the following month rather than retrospectively.
In personWho: You
5
Take the cross-border case to the GrenzInfoPunkt, then to an adviser
The GrenzInfoPunkt gives free, impartial orientation on which country taxes what and how it interacts with social insurance. It is not a substitute for a Steuerberater on a complicated year, but it will tell you what kind of adviser you need and stop you asking the wrong question expensively.
In personWho: You
6
File a return even if not required
A return frequently refunds — commuting costs, work equipment, relocation expenses, professional training and double-household costs are all deductible, and the relocation year especially. Cross-border years almost always require one.
OnlineWho: YouBy 31 July following the tax year

Documents you’ll need

Steuer-ID
Lohnsteuerbescheinigung — the annual employer certificate
A calendar of working days by country, for cross-border cases
Foreign payslips and foreign tax assessments
Records of deductible expenses, including the move and the commute

Things most newcomers don’t know

There is no frontier-worker shortcut with Belgium any more, and there never was one with the Netherlands.

The old German–Belgian frontier-worker rule, which taxed commuters in their country of residence, was abolished by the 2002 supplementary agreement with effect from 1 January 2004. Since then employment income is taxed where the work is done, under both the Belgian and the Dutch treaty. Advice written before 2004 — and a surprising amount of it is still circulating — is simply wrong.

Source: Bundesfinanzministerium — German–Belgian tax treaty and 2002 supplementary agreement

A Belgian-resident employee working in Germany gets 8% off their German wage tax.

Because those commuters remain liable to Belgian municipal tax while their wages are exempted from Belgian income tax, the treaty compensates by reducing the German wage tax withheld by 8%, which the employer notes on the wage tax certificate. It is a real and often-missed entitlement for the many people who live in Eupen, Kelmis or Raeren and work in Aachen.

Source: Bundesfinanzministerium — German–Belgian tax treaty

Exempt foreign income still raises the German rate on everything else.

Where a treaty exempts income taxed abroad, Germany applies Progressionsvorbehalt: the exempt income is added back purely to work out the percentage applied to your German income. People see 'exempt' and expect it to be invisible. It is not, and the effect on a partner's German salary can be significant.

Source: Bundesfinanzministerium — German–Netherlands tax treaty

Church tax is 9% in NRW and you opt in at registration.

Declaring church membership during the Anmeldung triggers automatic deduction of 9% of your income tax. It is decided in a short appointment most newcomers do not realise has a price, and leaving later requires a formal appointment and fee.

Source: Finanzverwaltung Nordrhein-Westfalen

Common mistakes to avoid

Relying on pre-2004 advice that says Belgian frontier workers are taxed where they live.
Not keeping a day-by-day record of which country you worked in.
Assuming treaty-exempt foreign income has no effect on your German tax rate.
Living in Belgium, working in Aachen, and never claiming the 8% wage-tax reduction.
Declaring church membership at Anmeldung without realising it triggers 9% church tax.

Some of this may be out of date. Spotted something inaccurate? Help us keep it right for the next newcomer.