Hello! I finally finished my third visa out of four! Phew!!
Awsome

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I am now looking ahead to save money for the last and final visa ILR. What is the full cost? I need to start saving now!
Currently the fee for ILR is £2,389. The fee hasn't risen for a while, but it could well go up in the next couple of years.
The good news is that there is no IHS surcharge for ILR so you don't have to worry about that extra cost.
Also, when I complete it can I qualify for dual citizenship?
If you are married to a UK citizen, you can apply for UK citizenship as soon as you have your ILR. You just need to make sure you meet the residency requirements for citizenship on the date you apply.
The citizenship residency requirements are:
- you hold ILR
- you were physically inside the UK on the exact date 3 years prior to your citizenship application date (i.e. say you submitted the citizenship application on 1st Jan 2021, you would have to show you were in the UK on 1st Jan 2018)
- you have not been outside the UK for more than 270 days in the last 3 years, and no more than 90 of those days can have been in the final 12 months before applying
When calculating days outside the UK, you do not count the day you left or returned, as you were inside the UK for part of those days. So, say, you went to France on Monday and returned on Friday, that would only count as 3 days outside the UK (Tue, Wed and Thu).
I am from the US and want to keep my US citizenship but have dual citizenship if that is possible.
Of course it is possible

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You can't lose your US citizenship unless you officially renounce it in front of a US government official... and it's very expensive to do that. It would cost you $2,350 if you wanted to renounce your US citizenship!
Officially though, the US sees you as a US citizen only, which means they will only acknowledge your US citizenship and not any other citizenship you hold... what this means is that you must always enter and leave the US on your US passport (and then use your UK passport to re-enter the UK), because it is illegal for a US citizen to enter the US on a foreign passport.