Running costs are the overheads you plan for, such as insurance, van finance, a phone contract or a storage unit. They are not money you have paid, so they are not expenses and they do not change the tax year. They are how you plan, not how you file.
Where to find it
Go to Finances, then Expenses & Running Costs, then Running Costs. It is a tab on the same row as Expenses.

What to enter
- Name: insurance, van finance, or whatever you call it
- Frequency: monthly, annual or a one-off
- Amount: what you expect to pay each time
- Starts: the date it begins
- Category: optional, the same categories you use for expenses
- Notes: the policy number, the lender, anything you would otherwise hunt for
Choose Add running cost and it stays on the list until you edit it and untick Active. When you actually pay it, record that payment as an expense. The running cost is the plan. The expense is the payment.

Common questions
Why is my profit unchanged after I add one?
Because a running cost is not a payment. Self assessment reads paid invoices, other income and expenses. A planned overhead is none of those until you pay it and record the expense.

Can I pause one?
Yes. Edit it and untick Active when it has ended. The history of what you planned stays, and it drops out of the list you are working from.