Self-Assessment is compulsory for those who carry on a trade or business as self-employed, or carry on a trade as a trader, these being limited partnerships. Also, those with rental income, foreign income or those individuals whose income is over £ 100,000 in a given financial year must file. Directors, those with certain levels of savings or investment income, or who claimed tax reliefs for everything from charitable giving to pension contributions, could be among those required to complete it. The rule of taxes applies to all and even though your employer will subtract tax for you through the PAYE system any extra income one happens to earn they will need to declare in order to meet their fair tax share.