Expat calculator — affordability and Lex Koller for newcomers
Foreign nationals wanting to buy residential property in Switzerland hit two hurdles at once: the banks' affordability rules and the authorisation requirement under Lex Koller. Both are documented separately and hard for newcomers to combine. The calculator answers both in a step wizard — no account, with a PDF to take away.
Verification as a product feature, not a footnote
A calculator that argues from law is only as good as its sources. We therefore checked every calculation rule against its source and documented the result in the repository: Lex Koller against BewG and BewV, equity and amortisation against the Swiss Bankers Association's minimum requirements. The fact check surfaced three errors in the Lex Koller logic that had looked plausible before. The second half matters more: the widespread rules of thumb — five percent imputed interest, one percent maintenance, one third of income — are not regulated figures at all but banking convention. In the interface they are therefore labelled explicitly as a market assumption, not as law. That distinction is the difference between orientation and misinformation.
Consent as compliance, not as a form field
The calculator generates leads, and leads go to two different recipient types — mortgage advice and brokerage. Consent for that is split in two and validated at the trust boundary, not in the frontend: a lead only reaches the recipient type actually consented to. Likewise the server route recomputes every input and stores only the server-side result; numbers from the browser are not trusted. Both are places where shortcuts are tempting day to day and expensive when it counts.
Status and what is still missing
The calculator runs, the engine is covered by tests, lead routing and PDF output work. What remains open we state just as plainly: the consent flow should get a legal review before we actively build reach. The subject-matter scope has limits too, and they are documented — the authorisation exemption for G-permit holders is modelled for EU and EFTA nationals, and second-home legislation (Lex Weber) is not covered. A calculator that does not know its own limits is more dangerous than none at all.