I do exactly what you want to do. I am employed and also freelance as a bookkeeper. You have to register your self employment with your local tax office. At the end of the year submit your accounts and you will pay tax in two instalments. If you are earning over the PAYE limit on your employment (which, as you are full time, is very likely) you will have no allowances to cover any of your self employed income and all your PROFIT will be taxed in full. Depending on how much you do in your self employment, it might be worth considering having your employment taxed at the basic rate. You will therefore have paid some of your tax before the end of the year. My income as self employed is significantly higher than employed, BUT i am only part time!! I would certainly make sure that you save enough to pay your tax bill. Put it in a saving account, get interest on it before you pay it over. If you are a good bookkeeper, you will not be short of work!