Compare ADU costs

Compare ADU costs

Side-by-side on the same six-phase ledger. Figures match the published posts — pick any two builds.

Build ABuild B

Build A · 800 sq ft detached

$320,000

$400/sq ft · 800 sq ft

Largest phase: Systems + interior ($147,000)

Modeled median · checked 5 Sep 2026

Build B · 400 sq ft detached

$215,000

$537/sq ft · 400 sq ft

Largest phase: Systems + interior ($102,000)

Modeled median · checked 5 Sep 2026

Difference (B − A)

−$105,000

+$137/sq ft

| Phase | 800 sq ft detached | 400 sq ft detached | Diff | | ---------------------------------------- | ------------------ | ------------------ | --------- | | Soft costs (design, permits, plan check) | $32,000 | $28,000 | −$4,000 | | Site work + foundation | $38,000 | $26,000 | −$12,000 | | Framing + structure | $48,000 | $26,000 | −$22,000 | | Building envelope | $42,000 | $25,000 | −$17,000 | | Systems + interior | $147,000 | $102,000 | −$45,000 | | Landscaping & hardscape | $13,000 | $8,000 | −$5,000 | | Total | $320,000 | $215,000 | −$105,000 | | $/sq ft | $400 | $537 | +$137 |

Half the size. Two-thirds the cost. Fixed costs — permits, the sewer connection, the kitchen, the bathroom — do not scale with floor area.

Soft-cost gaps also hide a legal cliff: many California impact fees change at 750 sq ft (seeMethodology/ Gov. Code §66311.5). Check whether a quote’s fee line jumps when size crosses that threshold.

Full post: 800 sq ft detached →Full post: 400 sq ft detached →

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Type what each contractor quoted for each stage and how it is listed. You get the gap against a published benchmark, the stages that are missing, and questions to ask before you sign.

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Compare against this benchmark

Benchmark: 800 sq ft detached, modeled median, checked 5 Sep 2026. Built from EPIC-LA permit valuations, LADBS permit records, NAHB phase cost shares. Not a quote. See Methodology.

| Stage | Benchmark | Quote A | Quote B | | ---------------------------------------- | --------- | ------- | ------- | | Soft costs (design, permits, plan check) | $32,000 | | | | Site work + foundation | $38,000 | | | | Framing + structure | $48,000 | | | | Building envelope | $42,000 | | | | Systems + interior | $147,000 | | | | Landscaping & hardscape | $13,000 | | | | Crane, transport & staging | $0 | | | | Total | $320,000 | — | — |

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Soft-costs-only is not a full build, so it is not a compare column. Download the blank ledger on theworksheetpage, or readwhy the six phases exist.