Normally, something significant like a retaining wall would be included within the application plans, and therefore should be permitted along with the house. Assuming that’s not the case, you can build a wall up to 2m high, or 1m if fronting a road, without the need for planning permission – fences or walls within these limits are so-called permitted development. The height should be measured from ‘natural ground level’. I can’t readily deduce from the information at hand how the wall and slope interact, but if you’re in doubt about the need for permission, do check either with the council or with your building designer. It will be better than having to break the wall down again at some later stage.