What sort of ground is under a Northampton garden?

It varies more here than most places. Northamptonshire sits on a mix of ironstone, limestone and clay, and which you have can change within a few streets. Clay is the one that matters, because it swells in winter and shrinks in summer, so the base has to be designed around that movement. Our buildings are light enough that ground screws often outperform a concrete slab, cost less and leave your lawn largely intact.