You need to file a personal tax return (SA100) if you were self-employed as a sole trader, a partner in a business partnership, a company director (in most cases), if you had rental income over £2,500, if you earned over £100,000 in the tax year, if you received untaxed income from savings, investments, or abroad, or if HMRC has sent you a notice requiring you to file one. You may also need to file if you received Child Benefit while earning over £50,000, if you made capital gains above the annual exempt amount, or if you want to claim certain reliefs or allowances that are not automatically applied. HMRC will usually write to you if they believe you need to file.
