Here in Delaware we can build a retaining wall without a permit if it is less than 48". How is this 48" generally calculated? Is it the height of the wall above grade or would it include any below ground measurement. In general if we built a wall with small boxes on top that we could put flowers or lights would that be part of the height calculation or just the part that holds back the dirt?
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Sometimes the height of a retaining wall can be hard to determine because of the segmental retaining blocks to build variable height or stepped walls on irregular ground.
Normally, for a retaining wall, the height is the maximum height of the face of the wall. It is usually treated this way to eliminate the loop-hole where say they will retain only 4' feet, but build a 6' wall in some areas. Then, the people seem to accumulate another 2 over time and they wonder why the wall fails.
How architectural/trim attachments are treated would depend on the local ordinace or code is written.
If you add a flower box on a legal wall, you should not have a problem.