Before your first payday, not after it. You need to register as an employer with HMRC once you take on anyone paid above the National Insurance lower earnings limit, anyone who has another job, or anyone receiving a pension — and directors count as employees for this. Registration takes a little time to come through, so leaving it until the week you want to pay someone means either a late first submission or an unpaid employee. You can pay someone below the lower earnings limit with no other job without a scheme, but the moment a second employee or a second job appears, you are in. If you have already paid someone without a scheme, register now and tell us — we can usually regularise it without penalties.