How Bilby works
Construction-loan interest and fees
Temporary credit has its own cost
The construction-loan row estimates interest and fees during construction. It does not add the loan principal to your build cost. Borrowed money pays for costs already elsewhere in the budget. The finished home’s monthly mortgage calculation is a separate view with separate inputs.
What is included
The reference includes interest on an assumed average drawn balance, an annual limit fee on the credit limit and a setup fee. It excludes loan repayment, plot and house purchase costs, mortgage-deed stamp duty and interest after completion. The rate is a planning assumption rather than a lender’s offer. Bank-specific inspection, valuation or other charges need reconciliation with the quoted scope.
Editable credit assumptions
Credit limit is not automatically the same as the future mortgage. Average utilisation expresses how much of that limit is used across the building period.
| Input | Values | Origin |
|---|---|---|
| Credit limit | SEK 2m / 3m / 4m | Assumed unless supplied |
| Duration | 6 / 12 / 18 months | Assumed unless supplied |
| Average utilisation | 40% / 50% / 65% | Assumed unless supplied |
| Annual interest | 5% / 6.5% / 9% | Bilby scenario rates |
| Annual limit fee | 0.25% / 0.5% / 1% | Bilby scenario rates |
| Setup fee | SEK 5,000 / 7,500 / 15,000 | Bilby scenario amounts |
The formula
Cost = credit limit × average utilisation × annual interest × months ÷ 12 + credit limit × annual limit fee × months ÷ 12 + setup fee. A supplied zero credit limit gives zero cost, including no setup fee. The reference retains arithmetic precision; the budget presents amounts in whole kronor. Each scenario combines its own inputs; low/high are not confidence limits around one fixed loan. An actual/manual total can replace this estimate, and an existing project amount is not silently overwritten by reference enrichment.
Fictional one-year build
Using the expected defaults: SEK 3,000,000 × 50% × 6.5% × 12/12 = SEK 97,500 interest. The limit fee is 3,000,000 × 0.5% = SEK 15,000. Add SEK 7,500 setup: SEK 120,000 total. The default low scenario is SEK 27,500; the default high is SEK 426,000. Their large spread also reflects different limits, utilisation and durations. At the expected assumptions, each additional month adds SEK 9,375 before any change in balance or rate.
Keep the finance inputs consistent yourself
A delay, larger credit limit or higher average use changes the result. House appearance does not. The own-contribution and monthly-mortgage controls do not automatically synchronise every credit-limit or mortgage-deed input. Review each finance row when financing changes. Missing values are explicit assumptions, not proof that the bank has granted that amount. Tax effects and a day-by-day drawdown schedule are outside this model.
Method and evidence
Bilby reference-cost V1, 2026 price assumptions researched 16 September 2026; implementation reviewed 18 October 2026. The banking source explains the construction-credit product, not a promise of the model’s rates. Obtain the lender’s actual terms. Swedbank
The review date covers this explanation and its implemented method. Price and data dates are stated separately in the article.