I've been communicating with my local authority, who clearly don't
understand the (prolly badly drawn) legislation.

Yep. The Department for Communities and Local Government appear to have
ignored requests for clarification on the issues you raise which were
made in response to the consultative document.

What I'm after is the definition of a 'deck' and 'ground'.

On "deck", the legislation actually refers to "a veranda, balcony or
raised platform". There's no definition of "raised platform". It
*might* be that it'd be construed in the light of "veranda, balcony " .
On the other hand it might be that "raised platform" just takes its
ordinary meaning (whatever that is). Either way I'd think you'd have a
pretty good case for arguing steps ain't a platform. But I ain't a
lawyer and what really matters is what the planning officer thinks.

If I built a deck over 30 cm high and built a raised bed level with
the edge, how big does that have to be to be classified as 'ground'?

The consultation document prompted some to comment that the rule on
decking needed careful drafting to avoid confusion; and that ground
level should take into account slope and pre-existing ground levels. So
you can tell how much the department was bothered. But your avoidance
device might work (unless of course the bed is over 300mm high and is
itself a "raised platform"?)

FWIW the ex. memo. to the regulations said that "Shayne Coulson at the
Department for Communities and Local Government (tel: 020 7944 8716 or
email: ) can answer any queries
regarding the instrument" so if you are feeling lucky you could ask
him. Of course the chances are he's moved on to a different job since
the regulations were made over a year ago

--
R