I'm trying to understand the size of your planting area. If it is a circle with a circumference of 5.5' the diameter (width) is only about 21". Is it is a 1/2 circle in shape? If so, what is the width along the side of the house?
Re. the depth. You state 2-1/2 ft. Is the height of the wall? or the depth of the soil? It’s important to realize that 90-95% of a tree’s roots are in the top 3 feet of soil and over half of these roots are in the top 12 inches and many roots extend beyond the longest branches a distance equal to 2-3 times or more the height of the tree.
Math isn't my strong point, but it seems like this is a quite small area for a tree. Can you provide more specific measurements?
1. Is there a hard bottom on the area, or could roots extend downward as far as they need to?
2. Width of the area along the side of the house.
Bear in mind that trees get tall. Conifers also get quite wide at the base. I'm guessing you don't want the tree rubbing against the side of the house or blocking your view. That's why it's important to carefully think about your planting area before selecting the appropriate plant, i.e., tree or shrub?
Also, you mentioned you would like a conifer--does that mean you wish year-round green as opposed to a shrub that drops leaves?
I appreciate your additional information.