When you rent a flat yourself, you have to pay council tax. The height of the tax depends on the flat (more specific the band of the flat, which is defined by the council) and the amount of rent you pay. Each flat belongs to a specific council tax band, which is represented by an uppercase letter. The earlier the letter in the alphabet the smaller is the percentage which defines the council tax. The smaller the percentage and the cheaper the rent, the smaller the council tax to pay.
(I am not an expert, so I hope this is all right but no responsibility is taken for the correctness of this information.)

More detailed information can be found on following websites:

• https://www.gov.uk/browse/housing-local-services/council-tax
• https://en.wikipedia.org/wiki/Council_Tax

Furthermore, the website of your borough (german: “Stadtbezirk”) might also provide some useful information.