The legal foundation: § 7 para. 4 sentence 2 EStG
In Germany, depreciation for rental property is usually calculated over a flat 40 or 50 years (2.5% or 2% annually).
The Income Tax Act expressly allows a shorter period if the actual economic useful life is shorter and this is demonstrated with a plausible expert assessment.
This is a legally provided right of choice, not a special exception.
“If the actual economic useful life of a building is shorter than the periods resulting from the standard rates, depreciation corresponding to the actual shorter useful life may be applied.”
Section 7 (4) sentence 2 EStG