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Once a year, on a date your mortgage deal sets: usually either 1 January or the anniversary of when the mortgage or the deal started. Unused allowance usually does not carry forward, so the date decides whether two overpayments a few weeks apart fall in one allowance year or two.

The arithmetic compares two rates. Each pound overpaid saves interest at the mortgage rate, with no tax on the saving; each pound saved earns the savings rate, which can be taxed. What the arithmetic leaves out is access: savings can be withdrawn, and money overpaid generally cannot.

The same total saves more if it goes in sooner, so a lump sum paid today saves a little more than the same money spread across the year. Over one year the gap is modest. Monthly overpayments keep the rest of the money to hand and make an allowance easier to watch; a lump sum has to exist first.

It depends on the balance, the rate, the time left and how early the money goes in. On £200,000 at 5% over 25 years, £100 a month extra saves about £24,500 of interest and ends the mortgage 3 years 6 months sooner; £200 a month saves about £41,800 and 6 years 2 months.

Of one of three things, and your deal picks which: the amount you originally borrowed, the balance on a fixed date each year, or the balance outstanding at the time. On the same mortgage those give materially different amounts, so the percentage on its own does not tell you what you can pay.

Either, and it is your lender's arrangement that decides which — not the overpayment. Shortening the term saves the most interest. Lowering the payment saves less but frees up money each month. Some lenders do one by default, some do the other, and some do nothing at all until you ask.

Less than you might expect at first, then more each year. At 5% over 25 years a £200,000 repayment mortgage comes down by about £4,100 in year one, £6,500 in year ten and £10,600 in year twenty, because the payment stays level while the interest inside it shrinks.

Usually because interest has been added and no payment has reduced it yet. Interest is charged on what you owe, so between payments, after a missed or partial month, or when a fee is added to the loan, the balance can rise even on a repayment mortgage.

On the balance you owe, at the annual rate spread across the year — by the day, the month or the year, depending on the mortgage. On £200,000 at 5% that is about £27.40 a day or £833 a month. The method rarely changes the payment, but it decides when an overpayment starts saving interest.

Not by themselves. A repayment mortgage payment is set so it stays the same for the whole term while the rate is unchanged. What falls over time is the share of each payment that is interest. The payment itself only moves when the rate changes, the balance is recalculated or the term is changed.

Because early in a repayment mortgage most of each payment is interest, not capital. The payment is level for the whole term, but the split inside it moves — and at the start it is weighted heavily toward interest, so the balance falls slowly even though you are paying in full.

Usually, yes. Fixed-rate deals commonly allow overpayments up to an annual allowance with no charge, and charge a percentage only on the amount above it. The allowance, what it is a percentage of and when its year starts are all set out in your mortgage offer.

As a percentage of the amount repaid early, at the rate in force on the day it is repaid. On an overpayment it is usually charged only on the part above the penalty-free allowance. When the whole mortgage is repaid inside the charge period, it is commonly charged on the whole amount.

To the lender, no: each borrower on a joint mortgage is responsible for the whole debt, whoever makes the payments. Who owns what share of the home is a separate question, settled by how the property is held and any declaration of trust, and later payments do not always change it. That is why a record of who paid is useful.

From three figures: what you owe, the interest rate and what you pay each month. Each month interest is added and the payment comes off, and the date is the month the balance reaches nothing. £150,000 at 4.5% with £1,200 a month clears in 14 years 1 month; at £1,400 a month, in 11 years 6 months.

Start with four figures: the opening balance, the interest charged, the payments received and the closing balance. The opening balance plus interest and any fees, minus payments, should equal the closing balance. If it does, the statement agrees with itself, and it is the figure every estimate should be checked against.

A spreadsheet tracks overpayments perfectly well until two people are paying into it. What breaks is not the arithmetic — it is that a spreadsheet has one copy, no record of who paid, and no memory of what it looked like last month.

The contractual payment goes up and the mortgage ends sooner. Because the balance falls faster, less interest builds up on it, so the total paid falls even though each payment is larger. It is a change to your contract, agreed with your lender, and the higher payment becomes the one you are required to make.

If the same money goes in every month, the arithmetic is identical: a 25-year mortgage with a regular overpayment ends on the same day, with the same interest, as a 20-year term paying the higher amount. The difference is the commitment. A shorter term makes the higher payment compulsory; an overpayment can be stopped.