Did the fence come up after this design? I do not see it lined in. It's a lovely design but there are a couple of functional questions or concerns I would have.
I am always concerned about blocking site coming out of a drive or street, so would question leaving the viburnum on the corner.
His choice of Spirea japonica lining your retaining wall...this species grows 4-6 ft tall and wide and likes full sun.
I really like liatris but to me, they are almost a specimen unto themselves. I would question the positioning under the dogwood as they also like full sun. However, he might be thinking that these would provide immediate attractive accents while the tree is growing.
The rhodis along the house I just don't see doing well in the soil conditions you described. Also the coreopsis and speedwell might be fine soil wise but will they survive the dogs?
We have Jack Russell Terriers. They have their own deck chairs and an entire part of the garden is devoted to their favorite sport...digging. It has compostable materials, dirt, and hollow logs for their enjoyment. No plants that I won't sacrifice are planted in this area. All other areas are "fenced" in some manner until established enough to survive the occasional bound and if more delicate than that, stay protected in some manner.
So with plants in the back yard where your dogs will be, I would make sure are tough and hardy. So, whereas I like Tom's suggestion of the Itea, it sounds more delicate than boxwood IMHO. And even the boxwood may not survive the urinalysis.
If your dogs eat stuff, I am definitely thinking the "herbs" around the A/C unit (is this the catmint?) are goners. I think your choice of hydrangeas would be fine or Tom's idea of the Ilex glabra for a variety of species within the yard.
The Star Magnolia...I love the idea, not sure it will love your soil conditions as they like "peaty, organic soil". And I am a big fan also of ninebark.
What did the landscaper suggest about the path on the property line for accessing with your lawnmower? I am still trying to come up with a ground cover suggestion that will allow some traffic, be attractive will blending in with the rest of the landscaping and providing erosion control.
Fun project, BB. In the landscape design classes I have taken, it is impressed upon us that there is more than one solution to a problem. Never one "right" answer. The "right" answer is the one YOU, the client, like and that suits your needs for the longest time possible always realizing gardens are growing and ever changing.
Sylvia