Buildable Area Calculator
The footprint a lot can take once front, rear and side setbacks are drawn in and the site-coverage limit applied — the envelope’s dimensions, its area and which rule governs.
Setbacks shrink the lot to a smaller rectangle: width less both side setbacks, depth less front and rear.
How the buildable area calculator works
Setbacks shrink the lot to a smaller rectangle: width less both side setbacks, depth less front and rear. That is the envelope. Zoning also caps the share of the lot the building may cover — 50–60% for houses, more in dense zones — so the footprint is the smaller of the envelope and the coverage limit. Floor-area ratios then cap the total floor space across storeys.
Formula: envelope = (width − 2 × side) × (depth − front − rear); footprint = min(envelope, lot × coverage)
Worked examples
| Inputs | Maximum footprint | Note |
|---|---|---|
| 15 × 40 m lot, 6/6/1.5 m setbacks, 50% coverage | 300 m² | envelope 336 m², coverage 300 m² — coverage governs |
| A narrow 10 × 30 m lot | 168 m² | envelope 168 m² governs |
| Setbacks that consume the lot | — | no envelope — refused with a note |
FAQFrequently asked questions
What setbacks apply?
The zoning or residential code for the lot: front setbacks often 4.5–6 m (or the average of the neighbours), rear 3–6 m, sides 0.9–1.5 m rizing with wall height. Corner lots, easements and overlays add more.
What is site coverage?
The share of the lot the building’s footprint may occupy, roofed area usually including garages and sometimes eaves and decks. It stops houses filling their lots and preserves space for landscaping and drainage.
How does the floor-area ratio fit in?
It limits total floor area across all storeys as a multiple of the lot; the footprint from this calculator times the number of storeys must stay under it. The floor-area-ratio calculator in the construction category does that check.
Can the envelope be irregular?
This calculator handles a rectangular lot; battle-axe, corner and angled lots need a drawn envelope. The arithmetic of subtracting setbacks is the same on each edge.
Where these figures come from
- Sabine (1922) — Collected Papers on Acoustics — the reverberation-time equation
- CIBSE Guide A — Environmental design — daylight, comfort and ventilation criteria
- Neufert — Architects' Data — the standard reference for space and dimension
- International Code Council — International Building Code — occupant load and egress factors
- International Code Council — publisher of the model codes most US jurisdictions adopt
Last checked: September 2026. Formulas are the standard building-science and code relationships; default thresholds are common international values and should be replaced with the ones in your code.