Published: 4 Sep 2026 · Last verified: 5 Sep 2026 · Next review: Dec 2026 Cost figures are re-checked against permit records quarterly. Changes are logged in Corrections.
Construction cost is the number everyone quotes. It is not the number that empties your account.
Start from the series baselines — $320,000 for an 800 sq ft detached ADU and $215,000 for 400 sq ft — then add the lines most articles skip: contingency, construction-loan interest, Proposition 13 tax on the ADU only, vacancy before the first rent check, and optional management. What you get is an all-in year-one picture, not a hard-cost fantasy.
The short answer
| All-in year-one · LA County models · 2026 | 800 sq ft | 400 sq ft |
|---|---|---|
| Construction (series baseline) | $320,000 | $215,000 |
| Contingency @ 12% | $38,400 | $25,800 |
| Construction loan interest (modeled) | $12,800 | $8,600 |
| Prop 13 ADU-only tax (first year, modeled) | $3,300 | $2,200 |
| Vacancy before first tenant (2 months modeled rent) | $6,400 | $4,800 |
| All-in year-one (self-managed) | ~$380,900 | ~$256,400 |
| Optional management @ 9% of year-1 rent | +$3,460 | +$2,590 |
| All-in year-one (with management) | ~$384,400 | ~$259,000 |
These are illustrative medians stacked on the published construction models. Your rate, draw schedule, assessed value, and lease-up speed will move every row. Treat the tables as a checklist with dollars attached — not as a quote.
What “all-in” means here
| Included in year-one all-in | Not included |
|---|---|
| Published construction total (six phases) | Land purchase |
| 12% contingency on construction | Major redesign / change-order blowouts beyond contingency |
| Modeled construction-loan interest during build | Permanent mortgage interest after refinance (separate analysis) |
| First-year property tax on ADU improvement only | Main-house Prop 13 base (unchanged) |
| Modeled vacancy to first occupancy | Ongoing annual OpEx after year one (insurance, maintenance reserves) |
| Optional year-1 management fee on rent | Furniture, staging, or luxury upgrades |
Soft costs are already inside the $320k / $215k construction totals ($32k and $28k respectively). Do not add them again.
Line 1 — Contingency (12%)
| 800 sq ft | 400 sq ft | |
|---|---|---|
| Construction | $320,000 | $215,000 |
| Contingency @ 12% | $38,400 | $25,800 |
| Construction + contingency | $358,400 | $240,800 |
Every post in this series says the same thing: budget 10–15% contingency as a line item, not as optimism. We use 12% here because it sits in the middle of that band and matches the language on the 800 and 400 posts.
Contingency is for soils surprises, long-lead equipment, inspector-driven changes, and the ugly week when the sewer lateral is deeper than the as-builts claimed. It is not a decorating allowance.
Line 2 — Construction loan interest (modeled)
Assumptions for this table — plain and checkable:
- Interest-only construction loan sized to construction + contingency
- 8.0% annual rate (illustrative 2026 working number; shop your actual quotes)
- 12-month average carry for the 800 model; same duration used for 400 for comparison
- Average outstanding balance ≈ 50% of the loan (draws ramp; you do not pay interest on the full amount every day)
| 800 sq ft | 400 sq ft | |
|---|---|---|
| Loan basis (construction + 12% contingency) | $358,400 | $240,800 |
| Avg outstanding @ 50% | $179,200 | $120,400 |
| Interest @ 8% × 12 months | $14,336 | $9,632 |
| Rounded model used in headline table | $12,800 | $8,600 |
The headline table uses slightly lower interest than the pure 50%-draw formula to reflect projects that close faster or draw late. If your build runs fourteen months or your rate is 9%, put your own numbers in the same cells. Modular delivery can shorten on-site months but may pull deposits earlier — interest timing changes even when the rate does not.
Line 3 — Prop 13 ADU-only reassessment (year one)
Building an ADU does not reassess your existing home. Only the new construction is added to assessed value. Your original Proposition 13 base year value on the main house stays put. That clarification is in every hard-cost post in this series; here we put a dollar on it.
| 800 sq ft | 400 sq ft | |
|---|---|---|
| Modeled assessable ADU improvement | ~$300,000 | ~$200,000 |
| Effective tax rate (illustrative ~1.1%) | 1.1% | 1.1% |
| Year-one tax on ADU only | ~$3,300 | ~$2,200 |
Assessor practice, exemptions, and local bond overlays vary. Some owners see assessed improvement closer to permit valuation; others see something nearer to cost. Verify with your county assessor — this row is a planning placeholder, not a tax bill.
Line 4 — Vacancy before first tenant
| 800 sq ft | 400 sq ft | |
|---|---|---|
| Modeled market rent (LA County ADU) | $3,200 / mo | $2,400 / mo |
| Vacancy before first tenant | 2 months | 2 months |
| Lease-up vacancy cost | $6,400 | $4,800 |
One to three months of vacancy after final inspection is common while you photograph, list, screen, and wait out notice periods. If you already have a tenant lined up (family occupancy, employer housing, pre-leased), this line can be near zero. If you finish into a soft rental market, it stretches.
Rent assumptions here are planning medians for mid-grade LA County ADUs in 2026, not a rent survey for your ZIP. The ROI post stress-tests payback against rent ranges.
Line 5 — Optional management
| 800 sq ft | 400 sq ft | |
|---|---|---|
| Modeled year-1 gross rent (12 months) | $38,400 | $28,800 |
| Management @ 9% | $3,460 | $2,590 |
Self-manage and this line is $0 plus your time. Use a property manager and you typically pay 8–10% of collected rent, sometimes with leasing fees on top. We show 9% on a full year of modeled rent as a clean optional add — even though real year one may collect fewer than twelve months.
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All-in year-one tables (print these)
800 sq ft detached — from $320,000 construction
| Line | Amount | Notes |
|---|---|---|
| Construction (six phases) | $320,000 | Full breakdown |
| Contingency @ 12% | $38,400 | Budget as a line item |
| Construction loan interest | $12,800 | Modeled; replace with your term sheet |
| Prop 13 ADU-only tax (year 1) | $3,300 | Improvement only; main house base untouched |
| Vacancy (2 months @ $3,200) | $6,400 | Lease-up |
| Subtotal — self-managed | $380,900 | |
| Optional management @ 9% | $3,460 | If not self-managed |
| Total — with management | $384,360 | Round to ~$384k |
400 sq ft detached — from $215,000 construction
| Line | Amount | Notes |
|---|---|---|
| Construction (six phases) | $215,000 | Full breakdown |
| Contingency @ 12% | $25,800 | |
| Construction loan interest | $8,600 | Modeled |
| Prop 13 ADU-only tax (year 1) | $2,200 | |
| Vacancy (2 months @ $2,400) | $4,800 | |
| Subtotal — self-managed | $256,400 | |
| Optional management @ 9% | $2,590 | |
| Total — with management | $258,990 | Round to ~$259k |
Side by side
| | 800 sq ft | 400 sq ft | Delta | | --- | --- | --- | | Construction only | $320,000 | $215,000 | $105,000 | | All-in year-one (self-managed) | ~$381,000 | ~$256,000 | ~$125,000 | | All-in ÷ construction | ~1.19× | ~1.19× | Same multiplier |
In these assumptions, all-in year-one sits about 19% above hard construction on both sizes. The absolute gap between 400 and 800 widens once carry and tax scale with the larger project — another reason to run both sizes through the same stack before you shrink the plan to “save money.”
Garage conversion and modular quick notes
- Garage conversion at $190,000: 12% contingency ≈ $22,800. Interest and tax scale down with the smaller basis; parking-replacement surprises often consume contingency faster than on a clean detached pad.
- Modular at $288,000: same 12% logic (≈ $34,600). Shorter on-site can cut interest; factory deposits and crane day overruns can offset it. Rebuild the interest row from your deposit schedule.
What breaks the all-in number
- Rate and duration. Fourteen months at 9% is a different interest line than nine months at 7%.
- Under-contingency. Bidding at $320k with 0% contingency is how people “discover” a $360k project.
- Assessed value surprises. If the assessor lands far from your permit valuation, year-one tax moves.
- Slow lease-up. Three months vacant at peak rent assumptions hurts more than one.
- Geography. Coastal vs inland changes the construction base before any of these add-ons apply.
Put your own quote in these six rows
We made a blank version of the six-phase ledger. Drop your contractor’s numbers into it and see which line doesn’t match ours.
Download the worksheet — CSV (import to Google Sheets), no email required
Want the regional benchmark ranges for each phase, updated quarterly?
Methodology and sources
Every figure on this page is a modeled median, not a quote. Here is exactly how it was built, so you can check it or reproduce it yourself.
| Input | Source |
|---|---|
| Statewide ADU rules, size limits, and fee exemptions | California HCD ADU Handbook |
| 2026 changes to State ADU Law | HCD Addendum, December 2025 |
| Declared construction valuations, unincorporated LA County | EPIC-LA permit portal |
| Declared construction valuations, City of Los Angeles | LADBS Permit & Inspection Report |
| Permit record dataset | LA County Open Data — EPIC-LA Permit Finder |
| Phase-level cost shares for residential construction | NAHB Cost of Construction Survey |
| Per-square-foot construction price benchmarks | NAHB Survey of Construction |
| Regulatory share of finished build cost | NAHB regulatory cost study, June 2026 |
| Zoning and parcel constraints | ZIMAS |
Check this number against your own address
Most California jurisdictions publish permit records online, including the declared construction valuation for each project. That declared valuation is the closest thing to a real local baseline you can get without paying anyone.
- Open EPIC-LA for unincorporated LA County, or the LADBS Permit & Inspection Report for addresses inside the City of Los Angeles.
- Search recent ADU permits within about a mile of your address.
- Compare the declared valuation to the phase totals on this page.
Declared valuations run below contract price — treat them as a floor, not a total. Full detail on Methodology.
If your local numbers diverge meaningfully from ours, we want to know. See Corrections.
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, 2026, stated as medians of contractor ranges. Costs vary by jurisdiction, site access, 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.
Frequently asked questions
What is the all-in cost of an 800 sq ft ADU in California?
Using this series’ $320,000 construction model plus 12% contingency, modeled loan interest, first-year ADU-only tax, and two months of vacancy, plan on roughly $381,000 self-managed in year one — about 1.19× construction under these assumptions.
Is contingency really necessary if I have a fixed-price contract?
Yes. Fixed-price contracts still exclude owner-driven changes, unforeseen site conditions in many cases, and items outside the contractor’s scope. A 10–15% owner contingency remains standard.
Will building an ADU reset Prop 13 on my whole house?
No. Only the ADU’s new construction value is added to your assessment. Your main house keeps its existing base year value.
Should I include management in all-in cost?
If you will not self-manage, yes — typically 8–10% of rent. If you will self-manage, put $0 and be honest about your time.
Where does rent payback fit?
All-in is the cost stack. Payback and cash-on-cash are the return stack — see ADU rent payback ROI.