An apartment can have three prices: in the listing, in the buyer’s mind and in the bank valuation. The first two happily talk to each other. The third arrives with documents and spoils the party.
My mortgage calculator therefore starts with distinctions easily lost in an impressive interface. Purchase price and bank valuation are separate amounts. So are the down payment and all the cash needed for the transaction. The monthly payment after the fixed period need not match the one the buyer likes today.
The calculator opens a fictional purchase scenario with separate price and bank-valuation fields. You can change inputs and examine how calculation works in this archived version.
Purchase price and bank valuation
Archived interface · demo data
Where the conversation with AI ends
I built the product with AI tools, but moved calculation into a separate TypeScript module. The React and TanStack Start interface passes inputs and displays results. A language model does not choose the payment amount by mood or fill in a missing formula with persuasive prose.
The engine separates regulatory rules from bank assumptions. They are different reasons for a result to change: a changed rule and a particular offer’s terms must not hide behind one “mortgage” constant.
After the fixed period, calculation continues with the outstanding balance and remaining term. You cannot simply replace the interest rate on the first line and pretend the earlier payments never happened. Affordability also has a stress scenario: seeing an uncomfortable figure before signing is useful.
The next screen opens additional parameters: income, obligations and calculation assumptions. Values are demonstrative; market terms belong to the saved calculator version.
Advanced purchase scenario
Archived interface · demo data
A link can reveal too much too
The calculator has Russian, English and Arabic interfaces. It also supports sharing a calculation. Copying a URL sounds simple, but putting every form field into it could send your income, debts and savings along with the apartment.
Those personal amounts are excluded from a normal link. Private sharing has a separate encrypted option. I like this detail: care for a person appears in what is absent from the address bar.
Market assumptions in the current source are dated August 2026. This is a snapshot of terms, not a promise of today’s rate. The tool helps prepare a conversation with a bank; it does not issue approval on the bank’s behalf.
This post concerns the calculator’s design; the mortgage article for buyers is separate. The same inputs must produce the same calculation. Assumptions are dated August 2026 and promise neither today’s rate nor bank approval.
Related stories: LiveDXB Analytics · Comps Engine research. I built a Dubai mortgage calculator: numbers first, apartment second.
@nevsky_alexander
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