How Bilby works
How Bilby calculates groundwork costs
Ground preparation beneath the house
Bilby estimates the preparation needed for the selected house and foundation, then reconciles it with terrain work. This is separate from the structural foundation. The budget can show both base preparation and the incremental effect of the chosen placement; these are coordinated scopes, not two complete earthworks contracts.
What belongs here
The native calculation covers machine mobilisation, soil excavation and replacement, suitable imported aggregate, delivery, disposal and compaction. Rock treatment, water management, investigation and weak-ground treatment are conditional. Vegetation clearance, demolition and temporary services have their own scopes. A supplier’s included groundwork package reduces the extra amount only according to its recorded coverage; a package description is not proof that every unusual condition is covered.
Quantities before prices
The selected foundation changes preparation depth and the portion of the footprint excavated. Soil evidence affects difficulty and reusable fractions. Foundation compaction and terrain fill lifts remain distinct quantities.
| Input | Unit | Where from? |
|---|---|---|
| Footprint and working area | m² | Selected house; ordinary working-area factor 1.3 |
| Soil and rock depths | m | Mapped/inferred evidence, not a site survey |
| Cut, fill, reuse and export | m³ | Scenario mass balance |
| Aggregate | tonnes | Imported volume × 1.7 t/m³ |
| Compaction | m² of lifts | Area and 0.25 m lift assumptions |
Reconcile masses and shared costs
Each scenario multiplies its own quantities by its own rates. Reusable site material reduces imports and exports. The combined method reconciles overlapping excavation, compaction and blasting setup instead of adding standalone estimates blindly. In the native 2026 version 2 model, normal soil excavation uses SEK 250 / 375 / 550 per m³; aggregate material uses 230 / 320 / 490 per tonne and delivery 100 / 175 / 275 per tonne. Reference fallbacks use a separate specification and can differ. Native totals round to SEK 5,000 below 100,000, SEK 10,000 through 500,000, and SEK 25,000 above that.
Illustrative aggregate component
For a fictional 10 m³ imported aggregate quantity in all scenarios, the native density gives 17 tonnes. Material plus delivery is 17 × (230 + 100) = SEK 5,610 low, 17 × (320 + 175) = SEK 8,415 expected, and 17 × (490 + 275) = SEK 13,005 high. Excavation, compaction and mobilisation are additional components; this example is before whole-estimate rounding.
What changes the result
Moving or resizing the house, choosing another foundation, new soil evidence and changing included contract scope can alter costs. A cosmetic house colour does not change these earthworks quantities. Missing terrain limits placement-specific quantities. An internal cost-burden classification is not a separate public suitability score or charge. Contractor access, contamination and disposal classification need actual investigation and quotes.
Evidence and version
Method review: 18 October 2026, native groundworks and combined mass-balance version 2, Swedish 2026 allowances. SGU maps inform soil evidence, not contractor prices SGU. Contractor machine lists illustrate separate plant and labour scope; Bilby’s unit coefficients remain modelled allowances, not a quotation from that supplier Sanmab.
The review date covers this explanation and its implemented method. Price and data dates are stated separately in the article.