Tax return (United Kingdom)

In the United Kingdom, a tax return is a document that must be filed with HM Revenue & Customs declaring liability for taxation. Different bodies must file different returns with respect to various forms of taxation. The main returns currently in use are:
Income tax self-assessment.
Most employees paying tax under the PAYE system are not required to file a tax return, because the PAYE system operates to withhold the correct amount of tax from their wages or salaries. However, some tax payers, including employees, may have income that has not been taxed at source and needs to be declared to HMRC, usually by submitting a self assessment tax return.
Legally, a tax payer is obliged to submit a tax return when HMRC request one by sending a notice to file a tax return, either because the tax payer has registered for self assessment voluntarily or because HMRC believe one to be required - HMRC can request a tax return from anyone for any reason.
Under UK tax legislation, tax payers are obliged to notify HMRC when they have a liability to tax no later than 9 months after the end of the tax year in which they became liable. Depending on the circumstances and the tax owed, they may do this by registering for self assessment and completing a tax return by January 31st..
Whilst there is no legal obligation to register for self assessment for income tax purposes when there is no liability for tax, HMRC guidance states that a tax return is required for the following reasons, some of which are not statutory:
The standard form in use is the SA100, complete with additional sheets for particular sources of income. A short tax return, form SA200, is available for those with incomes below £30,000. HMRC selects those who can complete a SA200.
The tax year runs from 6 April to 5 April. Tax returns must be completed by 31 January following the end of the relevant tax year for those who complete the tax return online and by 31 October following the end of the tax year for those who file by a paper return.
Once registered, tax payers can submit their tax return online directly via the HMRC website, or from online platforms.
Partnerships.
A partnership, including one in which all partners are companies, files form SA800. The partnership itself does not normally pay income tax, capital gains tax or corporation tax, but is required to provide a Partnership Statement to each partner reporting that partner's share of income and gains. Individual partners are also required to file self-assessment tax returns; although a partnership is not considered a separate legal entity like a limited company it is still a form of taxable business income. 
Trusts and estates.
A trustee, including trustees of certain pension schemes, must file form SA900 by 31 January following the end of the relevant tax year for those who complete the tax return online and by 31 October following the end of the tax year for those who file by a paper return. A personal representative administering the estate of a deceased person must file a form SA900 if the affairs of the estate are complex. Whether or not a tax return is required, each beneficiary's share of taxable income is reported to the beneficiary on form R185.
Corporation tax self-assessment.
A company must file a return, using form CT600, and assess its liability to tax, normally within 12 months of the end of its accounting year.
PAYE deductions.
At the end of the tax year, after 6 April, employers operating PAYE schemes must report to HMRC their employees, the total that has been paid to them, the amounts of income tax and national insurance contributions (NICs) that have been deducted from those payments, and the amount of employer's NICs due. This is done on form P35.