Yes you can be self employed and employed. Call HMRC to register as self emplyed and they will send the relevant tax return with the correct pages to complete. PAYE on one sheet and self employment income on the other. On your P60 it will show income and tax deducted just copy this on to tax return. The tax office looks at all your income as a whole applies the relevant tax coding credits you with tax already paid under PAYE and works out the bill. As for national insurance even though you you pay NI under PAYE you will have to pay additional NI under Self employment. It is a standard �2.60 per week and then a further 8% on any profit you make.If your self employment income is low (around 5400) you may be exempt.