The rules on tax residency in the UK are primarily determined by the Statutory Residence Test (SRT), which establishes whether an individual is a tax resident in the UK for a given tax year. The SRT consists of three main parts:

1. Automatic Overseas Tests
2. Automatic UK Tests
3. Sufficient Ties Test

1. Automatic Overseas Tests

You are automatically considered non-resident for the tax year if you meet any of the following conditions:

• Spent fewer than 16 days in the UK in the tax year (and were UK resident for one or more of the three previous tax years).
• Spent fewer than 46 days in the UK in the tax year (and were non-UK resident for the three previous tax years).
• Worked abroad full-time (averaging at least 35 hours a week) without any significant breaks and spent fewer than 91 days in the UK, of which no more than 30 days were spent working.

2. Automatic UK Tests

You are automatically considered UK resident for the tax year if you meet any of the following conditions:

• Spent 183 days or more in the UK in the tax year.
• Have a home in the UK (and you either have no home overseas or you have a UK home for a period of more than 90 consecutive days, spending at least 30 days in that home in the tax year).
• Work full-time in the UK for any period of 365 days, with no significant breaks and at least one of the days in the tax year being a working day.

3. Sufficient Ties Test

If neither the automatic overseas nor automatic UK tests are conclusive, the Sufficient Ties Test is used. This test considers the number of ties (connections) you have to the UK and the number of days you spend in the UK during the tax year. The ties include:

• Family Tie: Having a spouse, civil partner, or minor children living in the UK.
• Accommodation Tie: Having a place to live in the UK available for a continuous period of 91 days or more, and spending at least one night there.
• Work Tie: Working in the UK for at least 40 days in the tax year.
• 90-Day Tie: Spending more than 90 days in the UK in either of the two previous tax years.
• Country Tie: If the UK is the country where you spend the most days during the tax year (relevant for those who were UK residents in one or more of the three previous tax years).

Days and Ties

The number of ties required to be considered a UK resident depends on how many days you spend in the UK:

• Fewer than 16 days: Always non-resident.
• 16-45 days: Resident if 4 or more ties.
• 46-90 days: Resident if 3 or more ties.
• 91-120 days: Resident if 2 or more ties.
• 121-182 days: Resident if 1 or more ties.
• 183 or more days: Always resident.

Split Year Treatment

In certain situations, a tax year can be split into a UK part and an overseas part. This usually applies if you either:

• Start to live or work abroad full-time.
• Come to live or work in the UK full-time.

Domicile

Domicile is different from residency and relates to the country you consider your permanent home. It can impact how you are taxed on foreign income and gains.

Double Taxation Agreements

The UK has double taxation agreements with many countries to prevent individuals from being taxed twice on the same income.

In summary, the UK’s tax residency rules are complex and determined by various tests considering the number of days spent in the UK and other significant ties. It is advisable to consult a tax professional or use HMRC’s guidance for specific situations.