1. I regret to say that there is a requirement that your husband be visa free for one full year before he may make an application for citizenship. Accordingly, as your husband only got his permanent residence in January 2016, he will have to wait until January 2017 before he may make an application for UK citizenship.
2. The one year requirement of permanent residency only runs from January 2016, the date he got his PR. It does not run from before this time. So he will have to wait until January 2017 to get UK citizenship.
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4. By all means apply when he has one year of Permanent Residency. However, I would advise you to include this guidance from the Home Office Nationality Policy team if you make the application after one year of residency, as otherwise, you will be wasting an application fee. Be aware that whoever gets the application may not be aware of the ruling by the Policy Guidance unit, as there is always a discretion on any citizenship application. The requirement is to show you were "visa free" for one year.
5. The view that has been taken up to this is that you can only prove you were "visa free" for one year by proving it with your Permanent Residence card. So, if you hadn't got the Permanent Residence card, you could not prove you were visa free for one year.
6. I don't know where you will find Home Office Nationality Policy. You had better write to the Home Office or search their website. Be aware that any such policy statement does not have force of law. It is guidance to be followed. But it does not bind the Home Office.
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