GPT-5.6 Sol vs Kimi K3for everyday maths

Mortgage interest is charged on the amount you still owe. Overpaying early reduces that balance for many years, so you avoid interest on the overpaid amount throughout most of the mortgage. It also means more of each later payment goes toward clearing the remaining debt rather than interest. In the final five years, the balance is already much lower and there is little time left for interest to build up, so identical overpayments save far less.

Interest is charged on whatever you still owe. Early in a mortgage the balance is huge, so interest makes up most of each payment. An extra £100 then shrinks the balance, and that smaller balance keeps saving you interest every month for twenty-plus years — the savings snowball. Overpay near the end and the balance is already small and mostly principal; your £100 only reduces interest for a few remaining months, so there's barely any interest left to save.