This is covered in Article 14 of Schengen Visa Code. The required item stated as:
(b) documents in relation to accommodation, or proof of sufficient
means to cover his accommodation
The actual documents "in relation to accommodation" are explained in Annex II:
- for journeys undertaken for the purposes of tourism or for private reasons:
(a) documents relating to accommodation:
— an invitation from the host if staying with one,
— a document from the establishment providing accommodation or any
other appropriate document indicating the accommodation envisaged;
So technically a booking.com reservation - while often accepted - is not an obvious case. Because booking.com doesn't provide accommodation, it is is up to the Consulate to decide whether it is "other appropriate document" or not.
However a reservation booked directly on the hotel website would certainly qualify as "document from the establishment providing accommodation", so this is an option for you.
Finally, your other option is to provide "proof of sufficient, "means to cover his accommodation", such as banking statement.