Residents of Basel-Stadt are generally required to file an annual income and wealth tax return covering their worldwide income and assets (subject to treaty relief and allocation rules between cantons and countries). This page explains who must file, which deadlines apply, how the filing process works and how the rules interact with withholding tax (Quellensteuer) for foreign nationals.

Who Must File an Income Tax Return in Basel-Stadt?

You must typically file a tax return in Basel-Stadt if any of the following apply:

• You are resident in Basel-Stadt at the end of the tax year.
• You move to or from Basel-Stadt during the year and are resident for part of the year.
• You are self-employed or operate a business in Basel-Stadt.
• You hold a permanent establishment or partnership interest in Basel-Stadt.
• You own real estate located in Basel-Stadt (limited tax liability).
• You are subject to withholding tax and meet the conditions for a subsequent ordinary assessment.

Married couples and registered partners who live together usually file a joint tax return that includes the income and assets of both spouses/partners and any dependent children.

Residents vs. Non-Residents

The filing obligation depends on your tax status:

• Residents of Basel-Stadt generally declare worldwide income and assets, with relief for foreign tax under treaty rules and unilateral relief provisions.
• Non-residents with limited tax liability in Basel-Stadt usually declare only income and assets that are taxable in the canton (e.g. Swiss real estate or a permanent establishment located there).

Interaction with Withholding Tax (Quellensteuer)

Foreign nationals who do not yet hold a permanent residence permit are often subject to withholding tax on employment income. In Basel-Stadt:

• Your employer deducts tax directly from your salary using the applicable withholding tax tariff.
• You may still need, or be entitled, to a subsequent ordinary assessment in certain cases, such as:
  • If your income exceeds a specified threshold,
  • If you have significant additional income or assets,
  • If you want to claim deductions that are not fully reflected in the withholding tariff.

If a full tax return is required, the withholding tax already paid will be credited against the final tax due, potentially resulting in additional tax to pay or a refund.

Filing Deadlines

Basel-Stadt sets annual deadlines for filing income and wealth tax returns. In general:

• Tax returns are filed for each calendar year.
• The ordinary filing deadline falls several months after the end of the tax year.
• Deadlines may differ for individuals and legal entities.

Exact filing deadlines can change from year to year and may depend on whether you file on paper or electronically. Taxpayers who cannot meet the ordinary deadline can usually request an extension in advance.

Extensions of Time

If you are unable to submit your tax return by the ordinary deadline, you can generally apply for an extension:

• Extension requests should be submitted before the original deadline expires.
• Extensions are usually granted for a limited additional period.
• Further extensions may be possible in justified cases (e.g. illness, complex situations, delayed documents).

If you work with a tax advisor, the advisor may handle the extension process on your behalf in line with local practice.

Documents and Information Required

To complete your Basel-Stadt tax return accurately, you typically need the following documents and information (non-exhaustive list):

• Salary certificates (Lohnausweise) for all employments,
• Statements of self-employment income and business accounts (if applicable),
• Bank and investment statements as at year-end,
• Information on real estate in Switzerland and abroad (rental income, imputed rental value, mortgages),
• Evidence of deductible expenses (commuting costs, professional expenses where applicable),
• Proof of pension contributions and social security payments,
• Insurance premium statements (where relevant for deductions),
• Information on children and dependants (birth dates, custody, support payments),
• Details of foreign income and foreign tax paid (for treaty relief and avoidance of double taxation).

Depending on your situation, additional documents may be necessary (e.g. partnership or company documentation, trust or foundation information, cross-border employment certificates).

New Arrivals and Departures

Special filing considerations apply if you:

• Move to Basel-Stadt from another canton or from abroad during the year, or
• Leave Basel-Stadt during the year for another canton or another country.

In such cases, the tax year may effectively be split between cantons or between Switzerland and a foreign jurisdiction. Basel-Stadt will generally tax income attributable to the period of residence or limited tax liability in the canton. Your tax return must accurately reflect arrival and departure dates and any change of residence within Switzerland.

Self-Employed Persons and Business Owners

If you are self-employed or carry on a business in Basel-Stadt, you will typically:

• File a tax return including your business income and assets,
• Submit basic accounts (profit and loss statement, balance sheet) and supporting documentation,
• Be subject to both income tax and, where relevant, capital tax on business assets.

Filing obligations for self-employed persons are often more complex than for employees, making early preparation and professional advice particularly important.

Real Estate Owners

If you own real estate in Basel-Stadt, you will generally have to declare:

• Rental income or the imputed rental value of the property,
• Mortgage interest and permissible property-related deductions,
• The taxable value of the property for wealth tax purposes.

Even if you are resident in another canton or abroad, owning property in Basel-Stadt can trigger a filing obligation due to limited tax liability.

Consequences of Late or Non-Filing

Failing to file a tax return or filing late may result in:

• Reminder letters and administrative fees,
• Estimated assessments by the tax authorities,
• Possible penalties or interest on late payments.

It is therefore advisable to respect deadlines, request extensions when necessary and ensure that your tax return is complete and accurate.

Practical Considerations

In practice, many taxpayers in Basel-Stadt:

• Use the official tax software or online platform provided by the canton,
• Keep a dedicated file for tax-relevant documents throughout the year,
• Seek professional advice in cross-border or complex income/asset situations.

The filing requirements in Basel-Stadt should always be considered together with the applicable rates, deductions, special rules and examples to understand the overall impact on your personal tax position.