Legally, you may, so long as you disclose to your employer what you are doing (i.e. you let them know that you, an employee, are also billing for additional work): they are allowed to pay you more or on a freelance basis as well for any additional work.  Nothing prevents an employer from voluntarily paying additional amounts to an employee.However, your employer is not required to pay you for the additional work if you are salaried: your salary is yout *total* pay for *all* work you do for them. If you are salaried, that's all you get for all work done, including during your "spare time."If you are hourly, while they have to pay you for all hours worked (and overtime, when you work more than 40 hours in a week), they do not need to pay you other than paying your hourly wage. So while they may choose to pay you additional moneys or on a freelance basis, they do not need to. And if you do not disclose  that you are an employee when you bill them (e.g. you bill through an LLC and do not disclose your ownership), you will be committing fraud and they could sue you to recover any amounts they paid you which they did not have to.