@Poppy34 You don't (approach this with the guest). They are entitled to book your place at the specified price you have listed and the cleaning fee regardless of the purpose of use. And there is no way that you can legitimately check on what the property is being used for. It is a royal pain however when your sheets, quilt covers and soft fabric furniture ends up covered in fake tan and make-up. Or your nice clean sheets are stained with red wine either from spillage or vomiting.
We had a groom and his 3 groomsmen in a couple of weeks ago, I actually blocked the following night so they could leave to get to the wedding mid-afternoon without having to worry about late check-out and was happy to do this for their special day. No change in rate or extra charge from our end. 5 star review from the groom. They were extremely well behaved.
Another local host friend has many wedding bookings, but their rates don't change. So if you are likely to have quite a few wedding related bookings, then suggest you increase your cleaning fees instead and/or up your rates to cover all contingencies and "spread" any extra costs. There is no mechanism for "add-ons" subject to property use purposes. You can always claim additional expenses on the deposit feature you have activated although it seems getting Airbnb to actually do this is not easy.
I take it when you say "use on the wedding day" you are not talking about having the ceremony or celebrations there, just the preparation beforehand.
