In a proposed new garden there is no soil at all presently, just hardcore, stone etc, which will be excavated/moved to the required levels etc and then covered with fresh topsoil. I have access to as much good quality topsoil as I want with the additional option of manure etc as I need. Could you confirm the ideal topsoil depth for normal shrub, small tree, flower garden please?
It is likely to be subsoil as the topsoil was taken off prior to laying hardcore. The depth of hardcore will normally be no more that 200-250mm. The soil remaining needs to be broken up and not compacted. If the soil is very poor, you could take some of it away and replace with topsoil, if you have it plentifully.
In general, when a hardcore standing area is removed, the topsoil is replaced to bring it back to the same level. Lawns need about 200mm topsoil, flowers, 300mm, trees and shrubs, 300 to 400mm.
There is no need to add manure at the soil levelling stage, as it will rot away and the soil will sink by the same amount. Manure acn be added when the soil levels have been set. Allow about 10% for settling of soil. And make sure the digger works backwards, breaking any compaction in the subsoil as it goes back and does not compact the topsoil, especially in wet weather. Soil should not be moved when wet.