How this calculator works
Area = Σ (length × width) over the rectangles you typed.
What each input means
| Input | What it is actually asking |
|---|---|
| Rectangles | Split an L-shaped plan into two rectangles. Measure inside the walls. |
Worked examples
Including one where the naive answer misleads, which is the example most calculators leave out.
An L-shaped plan
Type the long leg as rectangle 1 and the short leg as rectangle 2, without counting the corner twice.
Just over 750
If the total lands just over 750, a small change can bring it under and remove impact fees in California.
What changes the result
Wall thickness matters near a line. Measure inside finished walls.
When not to use this
Do not use this for lot coverage, which measures the outside footprint. Use the size envelope calculator.
Questions this calculator answers
- Does a porch count?
- Usually not toward interior livable space.
- Does a loft count?
- It depends on local definitions and ceiling height.
- Why do the lines matter?
- They change fees and JADU eligibility in California.
Sources and methodology
Figures dated 27 October 2026. Last reviewed .
- Gov. Code § 66311.5 — 750 and 500 sq ft fee lines (California Legislature, retrieved 2026-09-05)
- Gov. Code § 66321 — the 800 sq ft floor (California Legislature, retrieved 2026-09-05)
Related
- ADU impact fee checker What the area means for fees.
- 500 sq ft ADU plans Why 499 is common.
- ADU floor plans Layouts by size.