When we built Mortgage Smart Schedules, we made a deliberate call: start with interest-only mortgages.

That wasn’t us being naive — it was us being realistic.

Most property investors we spoke to were using interest-only lending, so we optimised for the 90% case and kept things simple.

Then we started trialling the product with real portfolios.

And that’s when repayment mortgages showed up.

The messy reality of “simple” mortgages

On the surface, a repayment mortgage sounds straightforward. You pay one monthly amount and over time the loan goes down.

Under the hood though, it’s doing two things every month:

  • Paying interest on the remaining balance
  • Repaying a slice of the capital

From a pure accounting perspective, that splits neatly into profit & loss and balance sheet movements.

From a user perspective though… none of that should matter.

And honestly, that’s the tension we keep running into:

The more accurate the model becomes, the easier it is to make the product harder to understand.

We’re not trying to build an accounting textbook. We’re trying to help property investors understand what their portfolio is actually doing.

So we kept it simple (on purpose)

We extended Mortgage Smart Schedules to support repayment mortgages, but we didn’t change the mental model.

To the user, it still behaves like:

“This property costs me X per month.”

Even though, underneath, we’re carefully handling the split between interest and capital repayment.

That separation matters — but it stays out of sight.

Because if you have to think about double-entry accounting just to understand your mortgage… we’ve already lost.

A better way to explore the numbers

Mortgages aren’t intuitive.

Change the rate slightly, extend the term, adjust the loan amount — and suddenly the long-term picture looks completely different.

So we built something to help with that:

👉 Mortgage Playground

The Mortgage Playground is a space to safely experiment. Try different scenarios. Push the numbers around. See what actually happens.

No spreadsheets. No accounting logic. Just the reality of the loan.

Where this is heading

This is another small step in the same direction we keep coming back to:

Make the financial model accurate enough to be trusted — but simple enough that you don’t need to think about how it’s built.

Repayment mortgages were a good stress test of that idea, and they definitely made us think harder.

We think we’ve done a good job with this one — but we’d love to hear your thoughts.