How Bilby works
How slopes and house placement affect groundwork costs
The house position matters
Moving the same house changes which terrain cells lie beneath its pad, the elevation to build to and the amount of cut and fill. The selected-position groundwork estimate therefore differs from a plot-wide terrain score or an automatically suggested build area. Those are assessments, not your final construction quantities.
Incremental terrain work
Bilby compares existing elevations with a target formation surface. Cut is material removed above it; fill is material needed below it. The foundation profile adjusts excavation and levelling demand. Retaining needs, extra rock, imported general fill, exported spoil and compaction can follow. The combined calculation keeps engineered foundation fill separate from general terrain fill: reusable raw soil is not automatically accepted as structural aggregate.
Location, heights and material
Ground levels are sampled data, not a detailed contractor setting-out survey. Boundary coverage and missing samples limit the supported placement.
| Input | Unit | Where from? |
|---|---|---|
| House position, rotation, footprint | m, m² | Your selected geometry |
| Terrain cells and target surface | m elevation | Height data and grading method |
| Cut and fill | m³ | Depth difference × cell area |
| Reuse fraction | share | Soil-family/scenario assumption |
| Foundation type | system | Selection or disclosed fallback |
Mass balance before transport
For each scenario: reusable material = min(general fill demand, potentially reusable excavated material); imported general fill = max(0, general fill demand − reuse); exported material = max(0, unique excavation − reuse). Engineered foundation fill is added to imports separately. Cut soil and rock are split, and overlapping preparation/terrain work is reconciled. Fill compaction uses 0.25 m lifts, with shared first-lift overlap deducted where supported. A suggested formation search uses 0.05 m steps; the live placed-house grading can use a target surface over individual cells.
Illustrative mass balance
Suppose unique excavation is 40 m³, potentially reusable material 15 m³, general fill demand 25 m³ and engineered foundation fill 10 m³. Reuse is 15; imported general fill is 10; total imports are 20; exports are 25 m³. At 1.7 t/m³, imported aggregate is 34 tonnes. These fictional quantities demonstrate the balance, not a predicted volume for a particular slope.
Why flattening is not always cheaper
A lower pad can increase cut and rock; a higher pad can increase fill, transport and retaining. Reuse can create savings within the incremental slope result. Colours do not alter this balance. Missing terrain may leave placement-specific cost unavailable, while a separate reference remains visible. Scenario quantities and rates vary together; coarse total rounding and confidence envelopes can prevent displayed component sums from matching the rounded headline exactly.
Method and evidence
Reviewed 18 October 2026 against terrain build-area version 2, placed-house grading and combined groundworks. Geological input and pricing have different origins SGU. Bilby’s 2026 rates are planning allowances; surveyed levels and a materials-management plan are needed for procurement.
The review date covers this explanation and its implemented method. Price and data dates are stated separately in the article.