Whilst it sounds like the planned recess and 20cm gap above the Arc will be okay and might not affect the two upward firing channels in the Arc, which I think are set at 30° angles.. there are two side facing speakers that are designed to broaden the sound stage and it seems clear from your description that these will certainly be adversely affected… see this image:

So I would not place the Arc in a recessed area.

Hay thanks for your response! There will be 20cm clearance at either side too. Would you still advise against it?

That’s a slightly difficult one, as I’m fairly sure that standalone the Arc plays the front left/right channels and possibly the pseudo rear channels through its side facing speakers and certainly, when bonded with surrounds, those two side facing speakers will then add further/play just the front left/right channels ...and in both cases, they are at the very least designed to help widen the sound stage.
 

That said however, some users will undoubtedly have their TV/Arc placed on the diagonal in the corner of a room .. so I’m just trying to find for you the recommended clearance around the Arc and if I can find the answer, I will post back here. 

Okay I found this in the Arc Manual… see attached.

My own personal thoughts are you ‘might‘ just get away with 20cm either side, but I would perhaps choose to buy it direct from sonos.com as they offer a no quibble money back guarantee period and you can at least try the Arc in the recess and see how it sounds.

Hay, that's amazing, thanks so much for your help. The recessed shelf hasn't been built yet and I definitely have space to increase the clearance at either end to 30cm!