How to budget for one-off and unexpected costs

One of the most common reasons a budget stops working is not overspending on everyday items — it is costs that were always going to arrive but were never planned for. Car MOTs, annual insurance renewals, dental appointments, birthdays, school trips, boiler servicing. These are not emergencies. They are predictable. But without a plan, they arrive and eat into whatever is left at the end of the month.

The difference between irregular and unexpected costs

It helps to separate two types of non-monthly costs. Irregular costs are things you know will happen — you just do not know the exact date or amount. MOTs, car insurance, Christmas, school uniforms, replacing household appliances. Unexpected costs are the ones you genuinely could not have predicted: a medical bill, an emergency repair, a family crisis. Your emergency fund handles the unexpected. Your irregular costs fund handles everything else.

Write out your year in advance

The most useful exercise is to sit down once and write out every non-monthly cost you are likely to face over the next twelve months. Go through last year's bank statements if you can. Look at annual direct debits, seasonal spending (Christmas, summer holidays, back to school), and maintenance costs for anything you own. Add up the total, divide by twelve, and set that amount aside each month as a separate savings pot labelled "irregular costs".

If your total comes to £1,200 a year, you need to put aside £100 a month. That amount then sits waiting for each cost as it arrives, rather than being absorbed into day-to-day spending.

Use a sinking fund approach

A sinking fund is simply money you save in advance for a specific future cost. You can keep one general irregular costs pot, or run separate ones for things like car costs, holidays, and gifts. Many banks and savings apps let you create named sub-accounts or pots, which makes this easy to manage without needing a spreadsheet.

Build in a buffer for costs you forget

Most people underestimate their irregular costs the first time they work through the list. Add a 20 per cent buffer on top of your total to account for things you missed or costs that come in higher than expected. Over time, you will refine the figure as you get a more accurate picture of your year.

Review and top up quarterly

Set a quarterly reminder to check your irregular costs pot. If you have used some of it, recalculate what you need to put aside in the coming months to rebuild it. If a large cost is coming up — say a car service in two months — make sure the fund has enough to cover it.

How Ask Fin can help

My Monthly Budget in Ask Fin lets you include irregular savings as a line in your monthly plan, so the pot-building amount is treated like any other essential payment rather than optional saving.