I totally get your idea – it would be so useful for many!
Went to double-check with the devs, and, unfortunately, this feature cannot be customised for you to work as intended. The functionality is described in our Wishlist though, and has collected quite a few upvotes already: An option to set a buffer time between appointments. Drop yours too, and we’ll definitely consider the engagement in our development planning 
As for the workaround, there’s an option to change the visit duration to the “appointment time + gap“ in the widget settings, and we’ll be able to help you change the label to the “appointment time” duration. This way, your clients will see that they’re booking, for example, a 40-minute appointment, while actually booking a 50-minute one, including the gap.
However, the confirmation email will still state the “appointment time + gap“ duration. This peculiarity can be explained in the Description field.
Please let me know if you’d like our help with that, or if you have any more questions for me 