A second job may seem an attractive way of bringing in extra income, but are you allowed to by law, and do you need to tell your employers?

Is it legal?

There is nothing in law that stops you having a second job, although there are some things you need to be aware of:

• Working time regulations
• Tax implications
• Contractual requirements and conflict of interest

Working time regulations

The law states you may only work a certain number of hours each week (averaged at 48 hours a week), although you can choose to opt out of this if you want.

Hannah explains how she copes with working three jobs and studying at the same time.

"If you will be working over 48 hours because of a second job then your main employer will need to have a signed opt-out agreement from you and should monitor the hours you work in a week," says Sue Terry from the Advisory, Conciliation and Arbitration Service (Acas), who deal with employee-employer relations. "If they feel working long hours has an effect on your duties they may feel they need to speak to you regarding your performance."

If you are under 18 you can't work more than eight hours a day, and no more than 40 hours a week. You can't opt out of this.

Tax implications

Unfortunately, the taxman will still want to take his share of your wonga from your second job. You'll also have to make National Insurance contributions.

Because you won't have a P45, when you start your second job you'll need to fill in a P46. You can get one from your employer or download one from the HM Revenue and Customs (HMRC) website. Your employers will see you've declared that you have another job, but you don't have to tell them where you're working or how much you're earning.

Your personal allowance will usually only be used against your main job and tax will be deducted accordingly, although you can ask for your allowance to be split between jobs. You will have two tax codes - one for each job (so if you are really working hard and have three jobs, you'll have three codes!). It's important to check that the right tax code is being used for each, so have a look at your payslips If you have multiple tax codes, or you think you are paying too much tax, it's worth contacting HMRC to check you understand what's going on and that you are being taxed correctly.

Contractual requirements

You might like to believe what you do outside your working hours is up to you , but unfortunately it's not like that.

Your contract of employment may well have a clause preventing you from doing anything that could cause a potential conflict of interest or could bring your employer into disrepute. What your contract states will depend on where you work, but it's worth checking before you take up a second job.

Your employer is not going to be happy if you're doing some extra hours at a rival company and they are unlikely to look favourably upon you spending evenings as a lap-dancer if you're a school teacher by day. There could be legal problems if you are working for a company that supplies or buys from your employer, particularly if you are involved in the transactions. And whatever your day job is, don't be tempted to do work for job two while you're on job one's time, premises or equipment.

If your full-time role involves creating or inventing, whether that's graphic design, journalism or writing computer programs, you'll need to check your contract very carefully. It's not unusual to find a clause that says your employer owns all your creations of that type, even if you come up with them when you're nowhere near work. Some employers may be OK with you doing the same kind of work for other companies, but if they give permission, it's best to get this in writing.

It's worth being open with your employer about your other life; they'll often see it as a good way to develop your skills and keep you motivated, particularly if there aren't a lot of opportunities within your company. So if you're doing a spot of DJing, wedding photography or some bar work to bring in some extra cash your employer may well not see it as a problem.

Your employer can't simply bar you from taking a second job if there's nothing in your contract that prohibits it and there isn't any obvious deterioration in your performance. If they attempt to, it is worth getting specialist advice from Acas or your local Citizens Advice Bureau (CAB).

Updated: 19/10/2011