Break down the physical interchange
Start with the station where you will change. Crossing to the opposite side of one platform is a different task from navigating an underpass, several levels and a long concourse. Open the station map and look for the links between platforms, rather than simply locating their numbers. A lift may be at the other end from the staircase that appears to offer the shortest route.
Allow for getting off the first train. A passenger at the far end of a long train does not reach the platform exit the moment the doors open. Gathering several bags also takes time. Pack so that you can move everything in one trip without leaving possessions unattended or needing to return for a second load.
Separate transfer allowance from a planned break
Choose your connection around your walking pace, luggage and familiarity with the station. One universal number of minutes would be misleading. DB's journey search allows a longer transfer time. Compare the resulting journey as a whole: adding time at one station may produce a later arrival or an entirely different route.
Do not allocate the same minutes both to absorbing a small delay and to buying lunch or taking a necessary toilet break. If you need a substantial pause, select a connection that deliberately includes it. A waiting area with seats can be more useful on a family journey than the shortest technically possible interchange.
A station map alone is not enough when you need booked assistance or a particular lift. Confirm the arrangements with the responsible provider and review current notices. Assistance procedures, a lift fault or replacement transport may change an otherwise reasonable connection allowance. Treat those details as part of the route, rather than an extra task to resolve after booking.
Make a fresh decision on arrival
Check the current platforms for both trains shortly before arrival. Look at the connecting service itself, not just the delay shown for your first train. A platform alteration can make the walk longer even when the number of available minutes appears unchanged.
If the connection is no longer reasonably achievable, identify the next permitted option and ask about ticket validity when necessary. Running down steps with a heavy case is not a sensible way to rescue a very tight plan. After travelling, note the route you actually needed. That gives you a more useful basis for the next visit than a general rule about connection times.
