If its in your rules, guests have to leave at 12:00 or before! Otherwise they have to pay for the extra time and/or if you are having no other rooms and next guests are arriving at 14:00 they have to leave no matter what! At least you need time for cleaning the room, they will understand it.
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I for myself, remind my guests in the morning the day they are leaving and ask them friendly at what time they are planning to leave. If there are no other guests coming directly at 14:00 i let them know they can relax but i need an empty room by 13:00 sharp.
The best way to do it, is telling them a day before, that their holidays are finished and ask them about the time they are planning to leave, I also let them know i will bring them to the street and help them with luggage, its a good way also to check on the room, if something is broken or the keys are left behind and not taking with them.
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It will get easier with the time, dont worry and have fun 🙂
where are you renting out Darnesha?
cheers from Phuket
mos