Buying to let: stress-test the rent before choosing the mortgage
Rental income is not the same as disposable cash. Model vacancy, upkeep and financing conditions before relying on a headline yield.
A rental apartment is not financed like your own home
An attractive monthly rent can make an investment apartment appear to pay for itself. The lender, however, assesses a rental property differently from a main residence, and the owner must absorb costs that the headline rent does not reveal. UBS guidance reviewed on 16 October 2026 describes distinct financing principles and higher equity requirements for investment property. Obtain the lender’s current written criteria for the exact object rather than importing an owner-occupied mortgage calculator’s result. The useful first question is whether the property remains financially manageable under difficult conditions, not which advertised interest rate makes the optimistic case look best.
Build a cash-flow bridge from rent to reserve
Start with rent realistically collectible, then distinguish operating costs, maintenance, administration, insurance, financing costs, amortisation, tax and a reserve for vacancy or unpaid rent. Do not count a reserve as spendable surplus just because no invoice arrives this month. Likewise, mortgage amortisation reduces cash available even though it has a different economic character from an expense. Ask who pays which ancillary costs under the lease and which responsibilities remain yours. Avoid estimating every line as the same percentage of property value. A recent inspection and actual building records can reveal work that an average maintenance assumption would completely miss.
A hypothetical vacant-month test
Suppose annual rent is CHF 28,800. Assume, solely for illustration, CHF 7,200 of owner costs and maintenance reserves plus CHF 12,000 of interest, leaving CHF 9,600 before amortisation and tax. Two empty months reduce receipts by CHF 4,800, cutting that provisional amount to CHF 4,800. Add an unexpected CHF 6,000 repair and the simplified cash result becomes negative CHF 1,200 before the omitted items. These are invented figures, not market yields or lender rates. Their purpose is to show how several ordinary setbacks combine. A property can have an appealing gross yield while still requiring additional cash from its owner at an inconvenient moment.
Match the financing to the downside case
Ask the lender how rental income is recognised, what valuation is used, what equity must be available and which amortisation schedule applies. Test higher financing costs separately from lower rent rather than assuming the two risks cannot occur together. A fixed rate can make one cost more predictable but does not eliminate vacancy or repair exposure. A flexible rate can suit some circumstances while making reserves more important. Check restrictions on using pension assets instead of assuming a rental investment qualifies like an owner-occupied home. Discuss tax treatment individually; a general statement about deductibility is not a complete calculation of after-tax profitability.
Decide who will run the investment
Write down who handles tenants, emergency repairs, accounts and regulatory checks, and include the realistic cost of delegating those tasks. Review co-ownership documents and planned building work before signing. Keep a separate readily available reserve rather than investing every remaining franc into the purchase price. Compare the property with other uses of the same capital, including concentration and the time needed to sell. Ask Clearwise to compare financing on a consistent rental scenario with the weak case visible alongside the base case. An understandable investment should remain manageable without assuming uninterrupted occupancy, an immediate resale or permanent refinancing on today’s terms.
Sources and further reading
Original Clearwise analysis. External reporting provides context; rules and contract terms must be verified with the competent authority or provider. Some press articles may require a subscription.
Apply this to your situation
Compare your actual documents and clarify what matters before making a commitment.
Request a free consultationGeneral guidance, not a binding quote or personalised legal, tax or investment advice. Examples are hypothetical and results are not guaranteed.
