I got this email from German Revenue which did not directly answer my question on what rate to claim. But it's interesting otherwise. The guide they sent is attached to this post.
Domestic dividends and certain other capital gains are subject to an obligation to withhold capital yields tax in an amount of 25 % plus a solidarity surcharge of 5.5 %. Recipients of these payments based abroad can receive full or partial relief from deduction at source by means of an application on the basis of a double taxation agreement or Section 43b of the Income Tax Act (EStG) or Section 44a (9) EStG. This relief is achieved either by means of refunding the withheld taxes or exemption of future payments from tax withholding.
In order to claim for refund of German capital gains tax you must send a special application via normal letter post to us and we also need the original tax certificates on which is written how much capital gains taxes and solidary taxes you have paid.
The form
Application for refund of German capital income tax (de/en) is available in German or English (when you click the English or German flag) on the following website
www.bzst.de/EN > Private individuals > Capital Yield Tax Relief > Written application procedure > Refund procedure Section 50d (1) EStG > Refund forms Form 010005.
Only the yellow marked fields are mandatory fields.
In general, we need the certificate of residence on the application. But due to the Pandemic we also accept a separate original certificate of residence.
Please send the filled and signed application with the orignal signatures and the original tax certificate, on which is written how much capital gains tax and solidary tax you have paid, with normal letter post to:
Bundeszentralamt für Steuern
Referat St I B 3
53221 Bonn
Deutschland
The period for refund allowed for filing a request is
four years (cf. Income Tax Act § 50d Subsection 1 Sentence 9), and it commences at the close of the calendar year during which the returns on capital were received, i.e. were paid out to the creditor.
https://www.bzst.de/EN/Businesses/C...efund_procedure_pursuant_50d_1_estg_node.html
Note, we don't give acknowledgments of receipt and each application will be handled after the temporal receipt.
The withholding tax, which cannot be refunded, due to the Double Tax Agreement (15 percent), can be credited as paid taxes in your national income tax return.
Please find the explanatory notes (in this moment only in German) in the attachment.
The document "Important explanations" in English language is currently being revised.