Muck Away and Spoil Estimator

An extension dig produces far more spoil than most homeowners expect, and it does not all fit back in the hole it came from. A typical single-storey rear extension yields around 20-28 of excavated material once the topsoil strip and the foundation trench are added together, which usually means one to two grab lorry loads to cart away. This estimator turns your footprint and trench dimensions into a spoil volume, a spoil weight, and a grab load count, so you can budget the muck-away line before the digger arrives.

Enter the extension footprint, the topsoil strip depth, the foundation trench dimensions, and your soil type. The tool works out the in-ground volume, adds the swelling that happens when soil is dug loose, converts the weight into grab loads, and prices the removal at the current grab-hire rate. It splits the volume from the weight on purpose, because the lorry is filled by loose volume but charged by the tonne.

Why dug soil fills more than the trench

Soil compacted in the ground for years swells the moment a mini digger loosens it. That swelling is the bulking factor, and for subsoil it runs at roughly 20-30%. It is why people book a single grab lorry and end up needing two: the loose muck on the back of the lorry takes up more room than the neat trench it came out of. Weight is conserved when soil bulks, so the calculator applies the swelling to the loose volume that fills the grab, and works out the number of loads from the tonnage instead.

What a grab load costs

Muck away is priced by the grab load. An 8-wheel grab lorry carries around 16 to 18 tonnes of spoil, and hire runs at £150 – £420 a load including tipping. The calculator counts loads by weight and rounds up, because a part-load still costs a full load. Two things push the bill up: wet clay, which is heavier and bulks more, and anything that stops the spoil being clean and inert. Clean subsoil, clay, brick and stone go to a cheap tip, but made ground, contaminated soil, or Japanese knotweed-affected spoil cannot share a clean load and costs far more to test and dispose of lawfully. Whatever leaves site, ask the carrier for a waste transfer note and keep it for two years, because the duty of care sits with you as the waste producer.

A worked example

Take a 20m² single-storey extension with a 200mm topsoil strip and around 14m of foundation trench at 600mm wide and a metre deep. The strip and the trench together give the in-ground volume the calculator shows above. Add the 20-30% subsoil swelling and that becomes the loose volume the grab has to carry. By weight it lands near 20-28, which is one to two grab loads. At £150 – £420 a load, the muck-away figure is the range the tool prints, before you allow anything for wet ground or a load that turns out not to be clean.

A documented Oxfordshire extension struck a water pipe and hit made ground during its dig, both of which added spoil and unplanned removal that the first estimate never counted. Confirm in writing whether the groundworker has priced muck away inside the quote or left it to you: a labour-only price for the dig is the most common budget shock at this stage.