Grundheim’s Tranche Planner compares tranche structures made up of a ten-year fixed-rate mortgage, a five-year fixed-rate mortgage and SARON over ten years under six weighted rate scenarios for the SNB policy rate. You see the expected value of the interest costs for each variant, the costs year by year, the break-even policy rate, the insurance premium of locking in, the effect of the imputed rental value reform from 2029 and the cost of an early exit. The fixed rates follow the daily swap bands plus an editable margin; the result is a planning aid, not advice.
A rate scenario is a ten-year path for the SNB policy rate in quarterly steps. The planner works with six paths: the banks’ consensus (at zero for a long time, then 0.25 and 0.50 %), the path priced in by the swap market, an inflation regime with second-round effects, an early start because the SNB follows the ECB within months, a repeat of 2022 with 1.75 % within 18 months, and a franc shock with a return to negative territory. Each scenario carries a weight, and the weights add up to 100 %; the expected value is the weighted average, while the spread between the scenarios is the real message. Weights and anchor points can be changed freely, and three presets are available.
The fixed rate of a tranche is the CHF swap for its term plus a bank margin. The planner takes the midpoint of the daily swap band (rounded to five basis points) and derives the margin from the best published fixed rates in the Grundheim rate comparison, typically around 0.9 percentage points above the swap; without daily data, it uses the dated curve from the rate scenario. The SARON rate is the SNB policy rate plus a small spread (since 2019 SARON has traded slightly below the policy rate) plus the SARON margin, with the contractual zero floor. All three figures are input fields: if you have a bank offer, enter it.
For each variant and each scenario, the planner calculates interest costs quarter by quarter. The ten-year tranche stays at today’s rate, the SARON tranche follows the policy-rate path, and the five-year tranche is renewed in year six at the policy rate then prevailing plus spread plus margin. This produces the matrix views: the average per month over ten years, the most expensive year, years one and three, the total interest expense over ten years and the after-tax view. The expected-value row weights the scenarios; the colour shows which combination is cheap or expensive. Clicking a cell highlights the scenario and variant in all charts.
The break-even policy rate is the average policy rate over the term at which a fixed tranche and the SARON tranche cost the same: the swap plus fixed margin minus SARON margin minus SARON spread. If the average policy rate is above that, locking in was cheaper; otherwise SARON was. The insurance premium is the difference in expected costs between all fixed and all SARON over ten years: what predictability costs on a weighted-average basis. It depends heavily on the margin; an extra quarter point of margin costs 25’000 francs per million over ten years. Set against this is the repricing risk of 30 to 60 basis points when the market prices in an SNB hike.
Up to the 2028 tax year, mortgage interest is deductible at the marginal tax rate; one franc of interest costs net about 0.65 to 0.70. With the abolition of imputed rental value (popular vote on 28 September 2025, entry into force on 1 January 2029), the deduction disappears for owner-occupiers, with only first-time buyers keeping a temporary deduction that declines each year. The “after tax” toggle calculates this quarter by quarter. The carry of a SARON tranche is therefore tax-subsidised until 2028; from 2029 every amortisation becomes more attractive, and a variable tranche can be repaid whereas a fixed-rate mortgage cannot.
Anyone who terminates a fixed-rate mortgage before maturity pays the bank the interest differential: amount × (contract rate minus reinvestment rate) × remaining term, usually plus a processing fee. With a floor, the bank treats negative reinvestment rates as zero, which increases the compensation. The calculator in the planner shows the order of magnitude for the active variant; the contract formula is decisive. That is exactly why there is no stop-loss after signing: a fixed-rate mortgage is an insurance policy that cannot be cancelled without paying the premium afterwards.
The waiting window simulates the 10-year swap over the next 30 or 60 trading days: daily shocks from the empirical distribution since 2015, a jump on the next SNB decision day depending on the message (reassuring, neutral, concerned, normalisation hint) and a drift regime for energy prices. From 10’000 paths come the forecast band, the table of lock-in days with median, 80 % band and probability of being below today’s rate, plus the extra cost relative to locking in on the reference day. On the SNB day, the median swap moves by 2.5 basis points with no directional bias; the simulation is a distribution, not a forecast.
The swap bands are rounded market indications, and margins differ by bank, loan-to-value and negotiation. The planner does not know any amortisation schedule, fees or notice periods for SARON products, and the rate scenarios are Grundheim’s assessment, not a forecast. The curve is dated, and the SNB dates after 2026 are an assumption. Everything here is a planning aid for the conversation with the bank, not advice and not an offer.