If you are self-employed, you must keep a record of the running costs of your business throughout the tax year. Some of these running costs may be classed as allowable expenses. This means when you complete your tax return, these amounts can be deducted from your taxable profit for the year - meaning you pay less tax. Keeping good records (receipts and invoices) as proof of your costs is essential.
Knowing what you can and can’t claim as allowable business expenses can be confusing. If you are eligible, you could use the 'simplified expenses' scheme which uses a flat rate for things like vehicle costs or if you work from home. Watch this HMRC video which helps to explain the allowable business expenses you can claim on your self-assessment tax return and introduces the simplified expenses scheme.