You should not have:
1. Spent more than 450 days outside the UK during the 5 years before your application
2. Spent more than 90 days outside the UK in the last 12 months
3. Broken any UK immigration laws (for example living illegally in the UK)
You may have lost your indefinite leave to remain or enter if you’ve been away from the UK for:
1. More than 2 years at any time since you got it (you’ll need to apply for a Returning Resident visa)
2. More than 5 years if you have settled status
3. More than 4 years if you are a Swiss citizen, or the family member of a Swiss citizen, and you have settled status
As part of the eligibility requirement for applicants above 18, you will need to prove your knowledge of English, passed life in the UK test, evidence of intend to continue living in the UK & be of good character - read the naturalisation guidance