How Swiss Mortgage Affordability Is Calculated: The One-Third Rule Explained

Why Affordability Matters More Than the Purchase Price

In Switzerland, being able to pay the deposit is only half the battle. Before any bank or insurer grants a mortgage, they run an affordability calculation to make sure you can comfortably service the loan over the long term — not just at today's low interest rates, but at a higher, theoretical rate. This is a core part of Swiss lending culture, designed to keep household debt manageable and the property market stable.

For expats used to different systems (where affordability might be judged mainly on income multiples or debt-to-income ratios), the Swiss approach can feel stricter and more formulaic. Understanding it early will save you time and disappointment.

The One-Third Rule, Explained

The general principle — often called the "one-third rule" — is that your total housing costs should not exceed roughly one-third of your gross annual income. If they do, the property is generally considered unaffordable by the lender, regardless of how much deposit you can put down.

Housing costs, in this context, are not just your mortgage interest. Lenders typically bundle together:

• Theoretical mortgage interest — calculated at an imputed rate well above current market rates (often referred to as a "calculatory" or "affordability" interest rate). This buffer exists so that if interest rates rise significantly, you can still afford the loan.
• Amortisation — the portion of the second mortgage that must be paid down over time (more on this below).
• Maintenance and running costs — an assumed annual cost for upkeep of the property, usually estimated as a percentage of the property's value.

Add these three together, divide by your gross annual income, and if the result is above roughly a third, the bank will likely consider the purchase unaffordable — even if you have a large deposit.

Why an Imputed Interest Rate Is Used

This is the part that most surprises newcomers. Even if your actual mortgage rate is low, banks calculate affordability using a much higher hypothetical rate. The exact figure varies between banks and changes over time, but it is consistently well above prevailing market rates. The logic is straightforward: mortgage terms in Switzerland are often fixed for a few years at a time, and rates fluctuate over the decades you'll hold the loan. The bank wants assurance that you could still cover payments if rates rose substantially, not just under today's favourable conditions.

This is also why some buyers who look affordable at current rates fail the test — the calculation intentionally builds in a safety margin.

The Role of Amortisation

Swiss mortgages are typically split into a first and second mortgage. The first usually covers a large portion of the property's value and often doesn't need to be repaid on a fixed schedule. The second, smaller portion generally must be paid down — "amortised" — over a set period, often within about 15 years or by the time the borrower reaches retirement age, whichever comes first.

This required amortisation is added to your housing costs when calculating affordability. It's a real, ongoing cash outflow, and lenders factor it in just as they would interest payments.

Maintenance and Ancillary Costs

Banks also assume you'll spend a certain amount each year on maintaining the property — repairs, upkeep, and general wear and tear. This is usually estimated as a small percentage of the purchase price or official value, and added into the affordability equation alongside interest and amortisation.

What Counts as Income

Lenders generally look at gross household income, which can include salary, bonuses (sometimes only partially, or averaged over recent years), and in some cases rental income if you already own property. Self-employed applicants, and those with variable or foreign-sourced income, may face additional scrutiny or be asked for several years of documentation. Practices differ between banks and cantons, so it's worth asking early what income sources will and won't be counted.

Why This Matters Especially for Expats

Expat buyers should be aware that:

• Foreign income, especially if not in Swiss francs, may be assessed conservatively due to currency risk.
• Some lenders apply different rules depending on your residence permit type.
• Affordability calculations are largely standardised in principle but vary in detail from bank to bank, so it's worth comparing more than one lender or working with a mortgage broker.

The Bottom Line

The one-third rule is a useful mental model, but the real calculation involves several moving parts — an inflated interest assumption, mandatory amortisation, and maintenance costs — all measured against your gross income. Two buyers with the same deposit and purchase price can get very different answers depending on their income structure and the bank's specific assumptions.

Before falling in love with a property, it's worth getting a preliminary affordability assessment from a bank or independent mortgage advisor. This will give you a realistic price range and prevent wasted time on properties you may not qualify to finance.

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This article is general information only, not legal, tax, mortgage, or financial advice. Mortgage rules, calculatory interest rates, and lending practices vary between banks, cantons, and individual circumstances. Please consult a qualified mortgage advisor, bank, or financial professional, and confirm current requirements with the relevant Swiss authorities before making any decisions.