Groundworks: pricing the part of the job you cannot see

Everything above ground can be measured off a drawing. Below ground, you are pricing a guess about soil, water, roots, old footings and whatever the Victorians buried. The builders who make money on groundworks are not better guessers. They price the known, structure the unknown, and never sign a fixed price for a hole nobody has looked into.

Split the package into known and unknown

A groundworks price has two honest halves. The known half comes straight off the drawings: trench lengths, design depths, concrete volumes, drainage runs, slab areas. Measure it, rate it, done. The unknown half is the ground itself: whether the trench stands up or collapses, whether the digger hits clay, water, rock or an old air-raid shelter, and how deep building control actually sends you once they see the bottom of the hole.

The mistake is pricing both halves the same way. Fix the known half. Structure the unknown half with stated assumptions, provisional quantities and named rates, so that when reality varies, the price varies by a formula both sides agreed at the kitchen table, not by a negotiation over a muddy trench. The wording tools for this live in exclusions and assumptions and provisional sums.

Buy information before you sell a price

The cheapest risk reduction in construction is a trial hole. A morning with a mini digger, dug at the corners of the proposed extension, tells you the topsoil depth, what the subsoil is, whether water shows up, and how deep next door's footings sit. On sites with mature trees or known clay, a proper soil investigation is worth its fee many times over, because clay plus trees is how you end up at 2.5 metre trench fill or an engineered raft nobody priced.

Free information sources first: the neighbours' recent extensions (ask what depth they went to), the local authority building control officer's knowledge of the street, water authority maps for public sewers (a build-over agreement changes the foundation design), and your own eyes on the survey, which is why the site survey checklist puts drains, trees and levels near the top.

The quantities that move, and the factors that govern them

Bulking is the one that catches people over and over. Ten cubic metres of trench becomes thirteen in the pile, which at eight cubic metres a grab load is two loads, not one and a bit. Priced per load, counted from bulked volume, muck-away stops being a surprise.

Drainage: the other buried lottery

New drainage you can measure: runs, falls, manholes, connections. Existing drainage is the lottery. Old pipes turn up cracked, back-falling, made of pitch fibre, or exactly where the new foundations want to be. Camera surveys are cheap against the cost of finding a collapsed run after the slab is down, so on any renovation with suspect drains, either survey first or carry an explicit provisional sum for repairs and diversions. If the public sewer map shows a shared drain within three metres of the build, flag the build-over agreement early: it is the client's cost and the water authority's timetable, and neither should be discovered in week two.

Presenting an honest groundworks price

Structured this way, the groundworks section of your quote reads: firm measured items (excavation to stated depth, concrete, drainage as drawn, slab build-up), a short list of stated assumptions (ground bearing at design depth, no contamination, no obstructions, water table below formation), rates for the variables (extra depth, extra muck-away, rock or obstruction breaking), and provisional sums for anything the survey flagged as suspicious. Clients respect it, because every line answers the question "what happens if?" before it gets asked, and building control's decisions stop being your financial problem.

That is a lot of lines to assemble by hand, which is why groundworks so often ends up as one hopeful number. QS Quoter builds the whole section from your job description or drawings: excavation with bulking, muck-away by the load, concrete with overbreak, drainage, and the attendant items, each priced at your own rates and every line editable. The client copy shows a professional structured price; your private copy keeps every rate visible for the day the trench goes deeper.