Can I ask the hotel for things like a late check in or a twin bed when I book through your hotel booking API?

Yes, in most cases you can send a special request along with the booking, but it is worth understanding that a request is only ever a request and not a confirmed part of the reservation. The note is passed to the property, and whether they act on it depends entirely on the hotel and on how full they are on the day your customer arrives.

What you can send varies by supplier. Many suppliers accept a short free text note that reaches the hotel, so things like a quiet room, a high floor, a twin instead of a double, or a honeymoon mention can be passed along. Others only accept a fixed list of request types and will ignore anything outside it, and a small number accept nothing at all. Keep the wording short and in plain English, because some hotels receive these notes trimmed or translated inside their own system.

A late arrival note is the one we would always suggest you send, since a property that does not know your customer is coming in late may release the room. On the other hand, anything that costs money, such as an extra bed, early check in or a late check out, usually cannot be arranged through a request and will need to be settled with the hotel directly. For something a customer genuinely depends on, such as step free access or a ground floor room, do not rely on a note alone.

Before you promise anything, test a booking with a request on it and see what the supplier accepts, and always tell your customer the hotel will try rather than that it is confirmed.