German Citizenship

I hold a British passport and am 66 years old. I have no dependents.

In 2007 I purchased a house in Germany and lived there part-time, but maintained my main residence as well as my business in England.
At the time I was strongly advised not to register (Anmelden) as a resident in Germany as this would screw up my National Insurance contributions and therefore State Pension.

In 2011 I rented out my house in England, and closed down my business there. Since then I have lived more or less in my house in Germany. An average of 213 days per year in fact.

In 2018, upon reaching retirement age, meldete ich mich als Einwohner in meiner Stadt in Deutschland, and filled out the S1 form for the Krankenkasse.

All well and good, but it seems over on the Island they have completely and utterly lost the plot, by wanting to leave the EU. This, I am thinking, is going to cause me big problems. The solution - get German citizenship. Downloaded the bumpf and, yes, I could tick all the boxes.

But there's a problem - our local Einbürgerungsamt interprets "Rechtmäßiger gewöhnlicher Aufenthalt" as "Angemeldeter Aufenthalt". Without exception. So my questions are:

1) Is there anyone out there in the same boat?
2) Has anybody any experience with other Einbügerungsämter: it seems "Rechtmäßiger gewöhnlicher" is not defined as such in law, and a matter of local interpretation.

Well, by not registering your place of residence in Germany (which you had to, by German law - whether and how long you stay there per year does not matter) you broke German law and this was not detected (lucky you!).
Now you want this time recognised as residence for citizenship application purposes? Well, bad luck!
The only thing you could do now is to retroactively register (possible for up to three years a far as I know), hope that the authorities accept this (they are not obliged to), pay the fine for breaking the rules (which can be up to EUR5000) and hope that this fine does not affect your citizenship application. I would NOT do that without consulting a lawyer experienced in citizenship natters first!
Or you wait until you officially fulfill the residence periods required to become German. These can be shortened by showing good language skills and social integration. I would recommend this path!

Welcome to the Forum ajg1722

I am seeing more and more of your same case, not just in Germany but in Austria (where I am based) due to Brexit.

No simple way to say it but as beppi has stated you broke the law and no exceptions. They might take pity on you due to Brexit but still, expect a penalty because of the length of time and was prior to the 2016 Referendum vote.

Below I have posted 2 links please read them. other than that, I wish you good luck.

Living in Germany (UK Gov)
Brexit Germany

Beppi is right, you broke the law and it was your duty to be properly informed. I don't know who gave you such bad advice but it was completely naive to follow it. Germany has a totalization agreement with all EU and quite a few other countries that regulates contributions to the social system – so you would not have lost anything. You can of course apply for German citizenship - if you meet the requirements. Sounds like the time of residency will be a problem but there are a host of other requirements as well; passing the citizenship test, passing the B1 German language test for immigrants, proving you have filed and paid any due taxes and your contribution to the German Social Security system, lack of any serious criminal charges, evidence of your income, assets, accommodation and housing expenses, proof of marital status, birth certificate, passport and residency permit.... yeah they want to know a lot. And I'm up on the current situation because I just got my approval for German citizenship a couple of weeks ago as an American registered as living and working here for more than 20 years.

One also has to pay a 255 Euros fee and the waiting time is presently a minimum of a year or so, around 2 years for citizens of some countries.  Such things don't happen overnight and German immigration workers are over worked with the influx of refugees the last years and many Brits because of Brexit. Sorry to say but you are a bit late to start considering such things if you want to get German citizenship before the Brexit takes place, assuming it actually happens. A big advantage for the people who had the foresight to do it in time is that as long as the UK is in the EU then getting a German passport is possible without having to give up your current one. After Brexit I don't think there is any basis for this although Germany extends this possibility to a handful of countries like Argentina and Brazil because they are not allowed to renounce their citizenship.