Where is it permitted to flash headlights to indicate your intention to overtake?

You answered correctly. According to German traffic rules, you are allowed to briefly flash your headlights to signal your intention to overtake, but only outside of built-up areas, both during daylight and darkness. Within built-up areas (inside towns or cities), using headlights to indicate overtaking is not permitted, regardless of the time of day. This rule helps prevent confusion and unnecessary dazzling of other road users in densely populated areas. Remember: headlight flashing for overtaking is a communication tool specifically allowed only on country roads, not in city traffic.