Understanding
Why mortgage-free sooner may not mean more money
Becoming mortgage-free sooner, saving interest and ending with the larger modelled value are different measures.
Four outcomes to keep separate
Mortgage-free timing is when debt ends. Interest saved is the avoided borrowing cost. A projected investment asset is the value of contributions and assumed growth. Financial value compares the two paths at the same horizon.
Verified modelled scenarios
High mortgage rate
Scenario: £180,000 balance; 8% mortgage rate; 25-year term; £200 monthly spare cash; 7% assumed investment return.
Result: Mortgage-first leads by £100,384 modelled value.
Low mortgage rate
Scenario: £180,000 balance; 1.5% mortgage rate; 25-year term; £200 monthly spare cash; 7% assumed return.
Result: Investing leads by £66,364 modelled value.
Long remaining term
Scenario: £180,000 balance; 4% mortgage rate; 40-year term; £200 monthly spare cash; 7% assumed return.
Result: Investing leads by £143,876 modelled value.
Short remaining term
Scenario: £180,000 balance; 4% mortgage rate; 10-year term; £200 monthly spare cash; 7% assumed return.
Result: Investing leads by £400; the outcomes are close.
Lower or higher return assumption
Scenario: £180,000 balance; 4% mortgage rate; 25-year term; £200 monthly spare cash; return tested at 1% and 10%.
Result: Mortgage-first leads by £54,661 at 1%; investing leads by £109,574 at 10%.
Close outcomes
Scenario: £180,000 balance; 3.15% mortgage rate; 25-year term; £200 monthly spare cash; 5% assumed return.
Result: Investing leads by £98; a small modelled gap is not certainty.
Timing versus projected value
Scenario: £180,000 balance; 10% mortgage rate; 25-year term; £1,000 monthly spare cash; 2% assumed return.
Result: Mortgage-first clears the mortgage sooner and also produces the higher modelled value in this fixture.
What the engine does not decide
Access to money, moving house, approaching retirement, disappointing returns, changing mortgage rates, overpayment restrictions and early repayment charges can matter. The current engine does not calculate those edge cases; treat them as questions to investigate separately.