How Bilby works
How Bilby calculates foundation costs
Structure and soil preparation are separate
The foundation line estimates the structure supporting the house: slab on ground, crawlspace, piers or basement. Ordinary soil excavation, replacement and terrain adjustment belong to groundworks. Always check whether the house supplier includes the foundation before treating its displayed reference as an extra cost.
Different systems use different methods
The native slab calculation includes a fixed part, footprint area and external edge. Crawlspace uses a fixed part plus footprint area. The early engineering summary retains a provisional SEK 350,000 structural allowance for piers and basements; the project budget replaces this generic allowance with the selected system’s reference model. A basement reference includes its stated waterproofing and drainage scope. Do not add that scope again as an unrelated drainage allowance.
Reference system specifications
When a selected reference is used, each scenario is max(minimum, footprint × rate). The basement is an unfinished structure with slab, walls, waterproofing and drainage; excavation, blasting and interior fit-out remain separate. Slab and crawlspace native estimates, when available, retain their own formulas.
| System | SEK/m² footprint | Minimum SEK |
|---|---|---|
| Slab | 1,800 / 2,400 / 3,400 | 100,000 / 150,000 / 220,000 |
| Crawlspace | 1,600 / 2,300 / 3,500 | 100,000 / 150,000 / 220,000 |
| Piers | 1,100 / 1,800 / 3,000 | 60,000 / 100,000 / 160,000 |
| Basement | 8,000 / 12,000 / 18,000 | 500,000 / 800,000 / 1,200,000 |
Footprint, not total floor area
For a two-storey house, the ground footprint is not doubled. A complex outline may have a greater perimeter than a rectangle of equal area.
| Input | Unit | Where from? |
|---|---|---|
| Foundation choice | system | Project choice; disclosed slab fallback if missing |
| Footprint | m² | Selected house geometry |
| External perimeter | m | Exact outline, else 2 × (width + length) |
| Fallback perimeter | m | 4 × √footprint × 1.1 when dimensions unavailable |
| Supplier inclusion | scope | Recorded house package |
The native formula and fallback
Slab scenarios are fixed SEK 15,000 / 25,000 / 40,000 + area × 950 / 1,250 / 1,600 + perimeter × 700 / 1,000 / 1,400. Results round to whole kronor. Crawlspace is 35,000 / 45,000 / 55,000 + area × 1,100 / 1,400 / 1,700. When no retained native amount applies, the reference models use the greater of a minimum charge and area × rate. Reference rates are not additional native components.
Illustrative 10 × 14 m house
A fictional rectangular 140 m² slab has a 48 m perimeter. Native results are SEK 181,600 low, 248,000 expected and 331,200 high. Expected is 25,000 + 140 × 1,250 + 48 × 1,000. For comparison, the reference pier model at that area gives max(60,000, 140 × 1,100) = 154,000 low; max(100,000, 140 × 1,800) = 252,000 expected; max(160,000, 140 × 3,000) = 420,000 high. The systems are alternatives.
Reading the range
Area, edge length and system change the structure estimate. Soil and terrain mostly change the separate groundwork scope and recommendation. Small projects can be governed by a reference minimum. A generic or national allowance does not establish structural feasibility, moisture design or a supplier specification; use a scoped quotation to replace it.
Method and evidence
Reviewed 18 October 2026 against foundation structure version 2 and budget reference enrichment. The reference catalogue was researched 16 September 2026. These are Bilby consumer-cost allowances, not a verified current supplier tariff. Soil evidence has a separate origin SGU; a favourable soil map does not certify a foundation design.
The review date covers this explanation and its implemented method. Price and data dates are stated separately in the article.