Hi Chaps,
Being from an engineering background I must admit I'm not experienced in these quantity surveying standards, so I'll have to take a look at those when I have a moment free.
However, we have had a few requests like this one. For example another user wanted to have the area of excavation less than 0.5m, because they priced shallow excavations by m2 instead of m3. We've been thinking about how we can implement a solution that works for everyone and we're aiming to have that out in the first half of this year.
In the meantime there may be a trick we could use like TimO suggested , like offsetting the proposed or existing to get each layer volume. This would depend on the context, so if you have a current project where you need this feel free to email our support and we'll give you some feedback.
Leo