Published: 18 Sep 2026 · Last verified: 18 Sep 2026 · Next review: Dec 2026 Cost figures are re-checked against permit records quarterly. Changes are logged in Corrections.
This page is the Los Angeles County hub for Every Line Item’s California ADU series. It does not invent a new total. It points at the published series medians for an 800 sq ft detached, a 400 sq ft detached, and a garage conversion, then explains how City of LA vs unincorporated County permitting, soft costs, and the 750 sq ft impact-fee cliff fit on the same six-phase ledger.
Full method: Methodology. Side-by-side builds: Compare.
LA County ADU cost at a glance
All three medians below are Los Angeles County, 2026, mid-grade finishes, modeled on the same six phases. They are series figures from the linked posts — not a new county-wide average and not a proprietary estimating database total.
| Build (series model) | Total | Per sq ft | Soft costs | Full post |
|---|---|---|---|---|
| 800 sq ft detached, new | $320,000 | $400 | $32,000 | 800 sq ft ADU cost |
| 400 sq ft detached, new | $215,000 | $538 | $28,000 | 400 sq ft ADU cost |
| Garage conversion (~450 sq ft living) | $190,000 | ~$422 | $24,000 | Garage conversion |
| Soft-cost band (LA County mid-grade) | — | — | $28,000–$40,000 | Soft costs |
Half the floor area is not half the bill. The 400 model is two-thirds the 800 total ($215k vs $320k) because kitchen, bath, permits, and utility work barely shrink. That is the same pattern documented in the 800 and 400 posts.
Southern California site-built context used elsewhere in this series: roughly $375–$600 per square foot, with the coastal band toward the top. The LA County 800 median sits at $400 / sq ft inside that band.
City of LA vs unincorporated County
“Los Angeles County” is not one counter.
| Where the parcel sits | Typical permit path | What to open first |
|---|---|---|
| City of Los Angeles | LADBS plan check / permits | LADBS Permit & Inspection Report; zoning via ZIMAS |
| Unincorporated LA County (and many contract cities on the county system) | EPIC-LA | EPIC-LA portal; LA County Open Data — EPIC-LA Permit Finder |
| Other incorporated cities inside the county | That city’s building department (sometimes still EPIC-LA) | Local fee schedule PDF + that city’s permit portal |
This matters for schedule, fee worksheets, and which declared valuations you can search — not because this hub publishes a second set of City-only or County-only totals. The series medians remain the LA County mid-grade scenarios above. How those figures are anchored to public permit records is spelled out on Methodology.
Practical difference for a homeowner:
- Confirm jurisdiction (city vs unincorporated) before you download a fee schedule.
- Search recent ADU permits on the portal that actually covers your address.
- Treat declared construction valuation as a floor, not the contract price — same caveat as Methodology.
Six-phase ledger
Every build in this series uses the same six rows, in the same order:
| Phase | What it usually holds |
|---|---|
| Soft costs | Design, engineering, plan check, building permit, impact / connection fees |
| Site work + foundation | Excavation, footings, slab or stem wall, sewer lateral, utility trenching |
| Framing + structure | Lumber, framing labor, trusses, sheathing |
| Building envelope | Roof, windows, doors, siding, weather barrier |
| Systems + interior | MEP, insulation, drywall, kitchen, bath, finishes |
| Landscaping + hardscape | Paths, stoops, planting, parking replacement when required |
800 sq ft detached (series median) — from the 800 post:
| Phase | Cost | Share |
|---|---|---|
| Soft costs | $32,000 | 10% |
| Site work + foundation | $38,000 | 12% |
| Framing + structure | $48,000 | 15% |
| Building envelope | $42,000 | 13% |
| Systems + interior | $147,000 | 46% |
| Landscaping & hardscape | $13,000 | 4% |
| Total | $320,000 | $400 / sq ft |
400 vs 800 (same phases) — from the series comparison tables:
| Phase | 400 sq ft | 800 sq ft |
|---|---|---|
| Soft costs | $28,000 | $32,000 |
| Site work + foundation | $26,000 | $38,000 |
| Framing + structure | $26,000 | $48,000 |
| Building envelope | $25,000 | $42,000 |
| Systems + interior | $102,000 | $147,000 |
| Landscaping & hardscape | $8,000 | $13,000 |
| Total | $215,000 | $320,000 |
| Per sq ft | $538 | $400 |
Garage conversion (~450 sq ft living) — from the garage post: soft costs $24,000; site + foundation $14,000; framing $22,000; envelope $28,000; systems + interior $85,000; landscaping / parking replacement $17,000; total $190,000.
Hold any two of these models on one screen → Compare.
Soft costs & the 750 sq ft impact-fee cliff
Across the series, soft costs for typical Los Angeles County mid-grade detached ADUs land in a $28,000–$40,000 band. Published model points:
- 400 detached: $28,000 (13% of $215,000)
- 800 detached: $32,000 (10% of $320,000)
- Garage conversion: $24,000 (13% of $190,000)
Doubling floor area from 400 to 800 only adds about $4,000 in soft costs in these medians — design and plan check dominate more than square footage. Line-item ranges and jurisdiction notes: ADU soft costs.
The 750 sq ft cliff (state law)
Under California Government Code §66311.5 (see also the HCD ADU Handbook and Methodology):
- An ADU with 750 sq ft of interior livable space or less generally cannot be charged impact fees by a local agency, special district, or water corporation.
- Above 750 sq ft, any such impact fees must be charged proportionately relative to the primary dwelling — not as a full single-family fee.
- Impact fee ≠ connection fee or capacity charge. Water/sewer connection and capacity charges can still apply under 750 sq ft.
- School developer fees follow Education Code rules with a separate size threshold (commonly discussed around 500 sq ft). Do not assume the 750 rule covers schools.
In this series, the 400 model sits below the cliff; the 800 model sits 50 sq ft above it. In the soft-costs illustrative splits, impact / ADU / school fees only move about $2,000 → $3,000 between those models — a small step in the LA County mid-grade medians because design and plan check dominate phase one and many local ADU impact schedules were already compressed. In cities that still levy large proportional impact fees above 750 sq ft, the soft-cost jump can be larger than these medians imply. Always read the current municipal fee schedule PDF.
What moves the median
The same drivers appear across the series. None of these invent a new county total; they explain why a quote can sit above or below the published medians.
- Site access and utility distance. Long trench runs, panel upgrades, and awkward crane access move site work hard. The 800 post cites roughly $18,000 in Los Angeles County for a representative utility-distance package — and that line barely shrinks when the unit is smaller.
- Hillside / slope. Engineered footings, retaining, and access can add on the order of $20,000–$50,000 without changing the floor plan (same framing used in the 800 post).
- Coastal vs inland band. Same 800 sq ft drawings in this series: Inland Empire near $300,000 ($375 / sq ft), LA County median $320,000 ($400 / sq ft), LA coastal band near $380,000 ($475 / sq ft). Full table: Coastal vs inland.
- Finish grade. Builder-grade vs mid-grade across a full unit is typically $15,000–$30,000 in the series framing — one of the few levers you still control after permits.
- Modular vs site-built. Factory-built paths in this series have come in about 10%–25% below comparable site-built projects, with crane/transport offsets. See Modular vs site-built.
- Jurisdiction fee path. City of LA (LADBS) vs unincorporated County (EPIC-LA) vs other cities — fee schedules and review tracks differ even when state ADU law is the same. Soft-cost deep dive: permits and soft costs.
How to verify on your parcel
Do this in order — same discipline as Methodology and the soft-costs post:
- Confirm city vs unincorporated (and which portal applies).
- Download the current building permit fee schedule and any ADU-specific handout for that agency.
- Cross-check size / fee rules against the HCD ADU Handbook and Gov. Code §66311.5 for the 750 sq ft impact-fee line.
- Open EPIC-LA or the LADBS Permit & Inspection Report and search recent ADU permits near your address; note declared valuations.
- For City of LA parcels, check zoning / overlays in ZIMAS.
- Ask the counter (or your architect) for a written fee estimate before you lock a construction budget.
- Drop any contractor quote into the blank six-phase rows and compare to the series medians → Worksheet and Compare.
Declared valuations understate contract price. Use them as a floor and a relative signal — not as your all-in number. Broader owning costs: All-in cost. Rent math: Rent payback / ROI.
Next steps
| Goal | Where to go |
|---|---|
| Line-item 800 sq ft detached | 800 sq ft ADU cost |
| Line-item 400 sq ft detached | 400 sq ft ADU cost |
| Garage conversion phases | Garage conversion |
| Soft costs + 750 cliff detail | Soft costs |
| Coastal vs inland swing | Coastal vs inland |
| Same 800 sq ft build in San Diego County | San Diego ADU cost |
| Same 800 sq ft build in San Jose / Bay Area | San Jose ADU cost |
| Hold two builds side by side | Compare |
| How figures are built | Methodology |
| Contingency, carrying, year-one stack | All-in cost |
| Rent payback framing | ROI / rent payback |
Our position
We don’t build ADUs. We don’t sell plans. We don’t sell your information to contractors. No figure on this site has been paid for or reviewed by anyone whose costs we publish. More on About.
Figures are Los Angeles County series medians for 2026 unless a linked post states otherwise. Costs vary by jurisdiction (LADBS vs EPIC-LA and other cities), site access, hillside conditions, coastal review, and finish level. This is construction cost information — not a quote, and not legal, engineering, or financial advice. Confirm requirements with your local building department and a licensed California contractor.