Posted: 30/12/2015 at 16:17
Hi andi, welcome. I often get called in to situations such as yours and you've done a fine job so far. You've got structure, good design and primarily for easy care no grass.
I see you want roses and dahlias. Some would say neither are easy care, but pick the right ones, feed and prune correctly they're a doddle. Same with dahlias really. The greenhouse is in the right place so starting them off early will be easy. The only precondition for both though is to make sure they're both well fed. An annual application of well rotted manure across the open beds is all that'll be needed, with the odd liquid or granular feed as a post flowering boost.
You may want to consider the addition of a few perennials around the edges of the beds, bare soil isn't pretty, if you have a look on the RHS website go to perennials and have a look under planting types they list there the best plants for under planting roses. There's only one plant I'd say no to and that's alchemilla mollis. Very nice in the right place but once you have it it's a permanent resident.