Welcome to the forum

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Do you pay taxes through PAYE, or do you file your own taxes each year? Just trying to work out if you will be classed as employed or self-employed for the visa.
You must explicitly state your annual salary on the application form, and this figure will be matched to the annual salary stated in your employer letter, payslips and bank statements... so there's no way around it.
If you are classed as employed, then your employer letter must state ALL of the following:
- job title and current annual salary (must be more than £18,600)
- length of employment
- length of time earning current salary (must be more than 6 months for Category A)
- type of employment
If you are classed as self-employed, then you can only use the income earned (after expenses but before tax) in the last full financial year ... for example, April 6th 2017 to April 5th 2018 (anything earned since April 2018 cannot be considered, as the current tax year is still ongoing) There are a number of different documents that are required, including HMRC registration, annual tax returns, payslips, bank statements etc.