I work full-time for example I also work freelance and make money from freelance work. Do I need to…

Potentially, yes.

Employment income and freelance/self-employed income are treated separately for tax purposes. If your additional income means you need to complete a Self Assessment return, your salary, tax already deducted through PAYE and your self-employed income and allowable expenses would normally all need to be considered.

Keep records of:

freelance invoices; payments received; business expenses; equipment purchases; software subscriptions; relevant travel; other costs directly connected with the freelance activity.

Do not wait until the filing deadline to organise everything. Keeping the records properly from the beginning makes the return considerably easier.