I plan to work like 1 week/month in UK (self-employed) and live in Germany, do I have to pay taxes in Germany as well? if so, how much?
Let's say I am in the UK tax bracket below 32K GBP gross, hence 20% UK taxes.
Britain and Germany currently have a double taxation agreement which means you won't pay tax on the same amount of money in both countries but it's not a straight forward process and you have to declare everything in Germany to ensure it doesn't get "double taxed"
There is more information on the .GOV website regarding taxation in Germany, but overall the general recommendation in situations like this is to seek professional advice
There is also further information, including a brochure here
Hope that helps.
If you have income both in Germany and elsewhere, then the German tax office adds up your world wide income, calculates what your tax rate would be, and then applies that exact rate to your German income. So if you made €50,000 in Germany and €50,000 in the UK, they would calculate what percentage someone making €100,000 would pay, and take that percentage of your €50,000.
The UK, on the other hand, will calculate how much you would pay if all your income was in the UK, and then subtract what you paid in Germany.
If I understand you right and you make £32,000 in the UK and nothing in Germany, then Germany will make some calculation to get your tax rate, and you pay that tax rate applied to your German income of €0 - which is €0. The UK will calculate how much tax you should pay on £32,000 and subtract the €0 that you paid in Germany.