I have a corner very similar to this - shaded and constantly damp.(In my case because of a small spring). I have put in two different ligularias, some ferns, astible, alchemilla mollis, primula bulleyana, iris and hydrangea petiolaris as a background. They all do surprisingly well there and the ligularias are big and bold enough to almost fill the corner. None of these plants was very expensive and they all grow vigorously to fill the problem space.
Good luck!

The sun room should face south, NE is too cold in the UK. The idea is that you sit in the sun room out of the wind but in the sun.

Anyway, Honeysuckle is a woodland plant, so will do really well growing over the fence (with support)

I would cut down the hedge - it looks in your side of the boundary given the brown fence on the otherside of it. That will add a lot of sunlight and I think that you could treat it as a semi-shade area.

Your little summer house is so dinky! :-) The white picket fence is lovely too - but it really draws the eye to your problem corner - maybe not what you wanted?

I'd deffo cut back the hedge provided it's a species that will take it, and reduce the height. The bit we can see isn't 12' but is it Leylandii? They don't regrow very well if you cut into "brown" areas I understand. I'd get it down to a height you can trim without using a ladder even if it is Leylandii.

That small area is going to look very crowded if you go mad with your "Bold structural plants" - and if they're deep rooting they won't be happy with just 12" of soil.

Otherwise, it's down to what you like and what you want. It's all a matter of taste and your opinion is the one that matters.

Agree with previous comments re hedge,(about 3ft wide) we had leylandii and took them out completly and what a difference so much more light. Another idea Im using in damp areas caused by "run off" is to put in a land drain. or even dig a trench and fill with gravel this could possibly act like a sump to collect water away from soil.
Ive gone for marginal plants in damp areas, such as Gunnera Manicata can grow very big, very structural and Chamaeleon plant, astible.
very best to you.

This is an amazing post would be nice as a continuing blog.

Fatsia japonica and Crinodendron hookeranium will both do well in that situation. Both get large though, and not sure whether the roots will eventually find a way to escape the concrete base beneath, but if there's any cracks in it, they will cope quite well. Note that Crinodendron prefers non alkaline soil. Another one to consider which is large and architectural is Mahonia x media varieties, such as 'Charity'. If you want a Fatsia, wait till spring to get one, best planted then, not now.

I saw this and thought holiday chalet! So I think if this were mine I would turn it in to a beach hut and shingle area 'look'! with a hammock. I would bring the hedge down and pebble the area and add a couple of boulders, with 2 blue posts and a white hammock hung between. Lots of brightly coloured red and blue pots and some hanging baskets on the deck with a couple of striped deck chairs one in here and one on the deck. It would make a great summer haven. If you use a 14-20mm gravel you will have no cat problems.

I almost suggested pebble/ shingle for that small area beyond the white fence, and then realised that that is what Annie has just carefully (and probably with much effort) removed, so didn't! ;-). But it would make a good "foil" for, say, Fatsia or a bamboo in a pot.