@Joseph1741 If the guest has already booked, it would be a terrible idea to ask them for a fee when the exact dollar amount wasn't specified in the listing. That would come off as a bait-and-switch.
I don't think it's completely unheard of to make pool access an add-on amenity, but in your case I can't recommend it. In your Private Room listing, that pool/patio area is not just an extra bonus - it's your primary selling feature! With that in mind, you should assume that everyone who books will want pool access, and raise your base rate, if necessary, to cover the maintenance.