The answer is: there is no specified limit on absences. In other words, there is no minimum number of days, such as “180 per year”. The only rule is that the UK remains your main home and each case has to be assessed on its merit.
How does it work in practice? In our immigration practice we have to assess each case individually. For example, a case where a British spouse was sent, by their UK employer, to work in Hong Kong for 6 months, while remaining an employee for a UK company (and getting a salary in the UK and paying UK taxes). Yet it is a different thing when a British spouse was offered a job in Singapore for 1.5 years, for a Singapore-based employer and getting paid in Singapore.
Generally, you should maintain the UK as your home as much as possible. This can be achieved by keeping the bills and bank statements arriving to your UK address (including address of the parents if you didn’t keep your own place or had to stop renting while abroad). Another thing is the Financial Requirement. If you’re meeting it by using some or all earnings from your job outside the UK – it will be obvious you weren’t residing here. If you were working abroad for a few months while being paid a salary in the UK (to a UK bank account) then there is no problem with using a Category A or B (employment). Some other categories do not indicate where you were at all: savings or property rental income (other than the property where you are or will be living in the UK) can be in any country whether you’re residing there or not.
Other than the above, the more time you spend abroad the weaker your case will be, particularly if you spent more than 50% of the time outside the UK. On the other hand, this is a problem for Indefinite Leave applications (ILR, permanent residency), rather than for an extension because ILR is based on the residing for 5 years in the UK while extension isn’t based on any period of residency.
For the rules on this on the official website: https://www.gov.uk/guidance/immigration-rules/immigration-rules-appendix-fm-family-members
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