Those25

I'm in the process of building a raised patio. Am I right in saying that it has to be 150mm below damp course? So the patio won't end up being very high. I've watched some YouTube videos and most people seem to dig a trench around the edge, which I already have roughly marked out and lay concrete foundations and then use engineering bricks for the retaining wall. I'm looking for advise on the measurements of the trench (depth and width) and as I will have to dig this out and mark it out. I already have some ruble in the middle so will break this down for the base next. I plan on using Porcelain tiles to finish and a decorative stone brick to cover the engineer blocks. Any advise will be a great help as it's my first time doing this!