What are the rules on having a second job?
Generally permitted unless your contract says otherwise — but several things can restrict it, and the tax treatment catches people out more often than the employment side does.
What your contract may say:
An exclusivity clause, requiring you to work only for that employer. These are void and unenforceable in zero hours contracts and for low-paid workers, which is a specific protection many people do not know about.
A requirement to disclose or seek consent, which is common and which an employer should not refuse unreasonably.
A non-compete or conflict of interest clause, which is the restriction with real force — working for a competitor is a genuine problem.
Confidentiality obligations, which continue regardless.
What can restrict it even without a clause:
Working time. The combined hours across all jobs count toward the 48-hour weekly average, and rest break entitlements apply across the total. Employers are obliged to ensure compliance, which is why they ask.
Conflict of interest, which can be a disciplinary matter even if unstated.
Using the first employer's time, equipment or information, which is the usual actual complaint.
Fitness to perform, where a second job leaves you unable to do the first safely — particularly in driving, healthcare and safety-critical roles.
Sector-specific rules, including some public sector and regulated roles requiring declaration.
The tax position, which is where the confusion is:
Your personal allowance is applied to one job, normally the main one, through your tax code.
The second job is usually taxed at basic rate on everything with a BR code, with no allowance applied — which is correct if the first job uses the whole allowance and wrong if it does not.
You may end up over- or under-taxed if income shifts between jobs, and can ask for the allowance to be split.
National Insurance is calculated separately per job, which can mean paying less than someone earning the same in one job — or more, if both exceed thresholds.
Self-employed second income requires self assessment above the trading allowance.
General information, not legal or tax advice.