What this is about
People posted to Switzerland for a few years often pay twice: the home abroad keeps running, and a second one is added here. Swiss tax law has a deduction built for exactly that — the special professional expenses of expatriates. How it works in general is set out in our guide to the deduction. This article is about the canton of Basel-Stadt.
Who counts as an expatriate
Before the cantonal level matters at all, the basic condition has to be met. Art. 1 para. 1 ExpaV covers two groups: managers and specialists with particular professional qualifications who are posted to Switzerland temporarily by their foreign employer. Not everyone who comes to Switzerland on a fixed-term basis meets that test.
What "temporary" means is set out in Art. 1 para. 2 ExpaV: employment limited to a maximum of five years. And under Art. 1 para. 3 ExpaV the deduction falls away in any case where fixed-term employment is replaced by permanent employment — even if the five years have not been used up.
The legal basis in Basel-Stadt
For cantonal and communal tax, Art. 27 para. 1 lit. c of the cantonal tax act applies — the provision on the other costs required to carry out your profession. The special professional expenses of expatriates are given shape by the federal Expatriates Ordinance, which applies directly to federal direct tax.
Basel-Stadt publishes its own practice: information sheet on the special professional expenses of expatriates, Basel-Stadt tax administration (dated 6 January 2021, applicable from tax period 2021). You can therefore read in advance how the canton reviews the deduction.
How Basel-Stadt handles the deduction
Basel-Stadt has the clearest reasonableness rule of the whole series — and the most detailed documentation requirements. For a pharma and life sciences region that matters, because assignments are everyday business here.
The 20 percent rule: Basel-Stadt names a concrete reasonableness test
While most cantons only speak of "reasonable" housing costs, Basel-Stadt is specific: documented housing costs are generally accepted up to 20 percent of the annualised gross salary, but no more than CHF 48,000 per year.
That is a double limit, and both halves bite. On a gross salary of CHF 150,000 the limit is CHF 30,000 a year, so CHF 2,500 a month — not CHF 48,000. The cap only applies from an annualised gross salary of CHF 240,000 upwards. Pay more rent than that and you must expect the excess not to be recognised.
Also expressly regulated: if the home abroad is only kept for part of the period, the deduction is reduced pro rata temporis. And international local hires do not qualify as expatriates under the ordinance — being hired directly by the Swiss company, without an assignment, does not count.
What Basel-Stadt requires as evidence
The information sheet distinguishes expressly by the type of home abroad:
- Rented home abroad: a copy of the valid lease and a residence certificate
- Owned home abroad: a copy of the purchase contract and the land register entry — in addition, the wealth tax value and the imputed rental value of the owner-occupied property abroad must be declared
- Expatriate status: to be proven through the contractual basis — employment contract, assignment agreement, correspondence; the employer also adds a note under figure 15 of the salary statement
Expressly not deductible in Basel-Stadt are home leave costs during the stay. A tax service provided by the employer, on the other hand, does not count as tax advice under the exclusion list, provided it runs through the employer's payroll and applies to all employees.
And a rule that visibly changes the net effect: anyone claiming the special professional expenses of expatriates cannot additionally claim the standard deduction for general professional expenses — neither for cantonal tax nor for federal direct tax. Run the comparison only after accounting for that.
Example: the limit bites
A head of research with an annualised gross salary of CHF 150,000 rents in Basel at CHF 2,900 net plus CHF 190 utilities and carries the cost himself.
- Actual net rent: CHF 2,900 × 12 = CHF 34,800
- Reasonableness limit: 20 % of CHF 150,000 = CHF 30,000
- Generally recognised: CHF 30,000 — the excess CHF 4,800 is at risk
The limit depends on salary, not on rent. If you are still taxed at source, higher actual costs can be claimed through a subsequent ordinary assessment where its conditions are met — but the 20 percent rule still applies.
What to watch for in Basel-Stadt
Work out the 20 percent limit before you sign the lease. It depends on your annualised gross salary, not on the market price of the apartment — and it is the central checkpoint once housing costs exceed it.
Regardless of canton, it is almost always the same points that cost people the deduction: gross rent claimed instead of net rent, the wrong provision cited, rate determination overlooked when arriving or leaving mid-year, or the home abroad left undocumented. The details are in our guide to the deduction, and what applies to travel and relocation costs is in the article on those.
Who carried the costs is decisive
What matters is who bears the costs economically and how any employer reimbursement is treated for tax purposes. An actual reimbursement against receipts generally rules out an additional deduction. Where a lump sum is paid and added to taxable gross salary in the salary statement, a deduction remains possible.
Every case is different — the potential usually is not
Whether and how much you can deduct depends on your situation: how the assignment is set up, your employment contract, your housing situation abroad, and who actually carried the costs. This article sets out the principles and a simplified example; it does not replace individual advice. What does generalise: on fixed-term assignments, deductions are regularly left on the table — a second look is almost always worth it.