Many thanks to everyone for the replies. I do in fact have planning
permission for a raised patio though the design submitted was slightly
different from the version I'm thinking of now. I don't know if the changes
would be sufficient to warrant submitting an amendment, though.

The question about raised flower beds is an interesting one; I have two
such, also around 24 inches high, and I'm afraid it didn't occur to me to
seek permission to build them.