There are two basic routes to naturalisation for adults, both of which require you already have Indefinite Leave to Remain and do not have any unspent convictions.
If you are married to a UK national
You must have been lawfully resident in the UK for three years. This could be a combination of any legal stay, ie fiancé and spouse or any of the tier categories. Time spent in the UK as a visitor will also count. You must also meet the residence requirements, and will not normally have spent more than 270 days out of the UK in the last 3 years and not more than 90 days out of the UK in the last 12 months. There is a little flexibility on this if absences were for work or compassionate reasons. There is no need to have had Indefinite Leave to Remain (ILR) for a clear 12 months.
If you are not married to a UK national
You must have been lawfully resident in the UK for at least 5 years and must have had ILR for at least 12 months. You must have not have spent more than 450 days out of the UK in the past 5 years and must have not been absent from the UK for more than 90 days in the past 12 months. Again, there is a little flexibility on this.
If you were granted ILR outside of the immigration rules, for example as part of the ‘legacy’ scheme, then the Home Office may decide to waive the requirement to have been lawfully resident. This would usually require submissions on the compassionate grounds, and the Home Office will take into account the length of stay in the UK, family ties in the UK and whether or not you have any family in your home country. Although naturalisation is not guaranteed it may be that an application is worthwhile. This should be discussed with an advisor.
As of 6th April 2011, an application will not be approved if the applicant has an unspent criminal record, and the ‘Rehabilitation of Offenders Act’ should be looked at to see if a conviction is considered ‘spent’ or not. Although Fixed Penalty Notices are not considered to be criminal convictions, having more than one in the qualification period may be a barrier to approval. If someone has a conviction for a motoring offence this may not be considered as a criminal record if, for example it is merely for speeding, however it should be declared on the application. Offences such as drink-driving and driving without insurance are considered serious and will result in a criminal record.
For further information on Naturalisation and Nationality