Before you start

A National Insurance number
PAYE registration through your employer
A Government Gateway account, if you ever need Self Assessment
Records of foreign income and overseas accounts

Step-by-step

1
Check for the C prefix on your first payslip
A Welsh taxpayer's PAYE code begins with C — for Cymru. If yours does not, and you live in Wales, HMRC has the wrong address for you, and it is worth fixing early even in a year when the Welsh and English rates are identical, because the Senedd can set them apart. An S prefix means payroll has you down as Scottish, which is a straightforward error.
Via employerWho: YouFirst payslip
2
Understand what 'Welsh rates' actually means
The UK rates are reduced by 10p in each band for Welsh taxpayers, and the Senedd sets a Welsh rate to add back. Every year so far it has set 10p, so the total matches England's. It is a live devolved power, not a formality, and it is decided in the annual Welsh budget.
OnlineWho: You
3
Know that residence, not workplace, decides it
HMRC treats you as a Welsh taxpayer based on where your main home is, not where your employer is. Living in Cardiff and working in Bristol makes you a Welsh taxpayer; living in Bristol and commuting to Cardiff does not. If you move across the Severn mid-year, tell HMRC.
OnlineWho: You
4
Learn the bands that currently apply
A personal allowance, then Basic, Higher from £50,270 and Additional from £125,140 — the same thresholds as England, unlike Scotland's six bands. The thresholds have been frozen for several years, which quietly pulls more people into higher bands each year as wages rise.
OnlineWho: You
5
Understand the 60% trap above £100,000
The personal allowance is withdrawn by £1 for every £2 earned above £100,000, giving an effective marginal rate of about 60% between £100,000 and £125,140. This is a UK-wide mechanic and applies in full to Welsh taxpayers. Pension salary sacrifice is the standard response.
OnlineWho: You
6
Check whether you actually need to file Self Assessment
Most employees never do — PAYE handles it. You must register if you are self-employed, have significant untaxed income, or meet one of HMRC's listed triggers. Online returns are due by 31 January following the 5 April year end, and you file to HMRC, not to the Welsh Revenue Authority — the WRA collects Land Transaction Tax and landfill tax, not income tax.
OnlineWho: You

Documents you’ll need

National Insurance number
P60 — the annual summary from your employer, showing the C-prefixed code
P45, if you change employer during the year
Records of foreign income and overseas accounts
Government Gateway credentials, for Self Assessment

Things most newcomers don’t know

Your tax code starts with C, and almost nobody notices until it is wrong.

The C prefix marks you as a Welsh taxpayer. Because the Welsh rates have so far been set to match England's, an incorrect code costs nothing today — which is exactly why it goes unfixed for years. The moment the Senedd sets a different rate, a stale code becomes a real error on every payslip. Check it in month one, when it is a two-minute call.

Source: GOV.UK — Welsh rates of income tax

Where you sleep decides your income tax; where you work decides nothing.

HMRC assigns Welsh taxpayer status by main residence. Cardiff and Bristol are a forty-minute commute apart with no toll on the Severn crossings since December 2018, so a lot of households straddle the border — and each mover has to tell HMRC, because the wrong side means the wrong code and, if the rates ever diverge, the wrong tax.

Source: GOV.UK — Welsh rates of income tax

The 60% band between £100,000 and £125,140 is the biggest hidden rate in the system.

The personal allowance is withdrawn at £1 for every £2 above £100,000, so each extra pound in that range is taxed at the 40% headline rate plus the lost allowance — around 60% in effect. It applies to Welsh taxpayers exactly as to English ones. Model it before accepting an offer in that range and look at salary sacrifice.

Source: GOV.UK — income tax rates and personal allowances

The tax year runs 6 April to 5 April, which catches everyone once.

Almost no other country uses these dates. It decides which year your arrival income falls into, when your P60 arrives, and when Self Assessment is due. Newcomers routinely reckon against a calendar year and get their first filing wrong.

Source: HMRC

Common mistakes to avoid

Not checking that the PAYE code carries a C prefix when you live in Wales.
Assuming a Welsh address means lower income tax — the rates currently match England's exactly.
Moving across the Severn without telling HMRC which side you now live on.
Accepting a salary between £100,000 and £125,140 without modelling the 60% effective band.
Counting the tax year as January to December rather than 6 April to 5 April.

Some of this may be out of date. Spotted something inaccurate? Help us keep it right for the next newcomer.