Methodology
Every cost figure on this site is a modeled median built from public data. It is not a quote, and it is not an average of quotes we received. This page explains exactly how the numbers are produced.
The unit of analysis is one specific scenario
We never publish a range as a headline. Each post models one defined build: a fixed square footage, a fixed jurisdiction, a fixed construction type, a fixed finish grade, and a stated site condition.
That is what makes the number checkable. A range cannot be wrong. A specific scenario can.
The six-phase ledger
Every build is priced into the same six phases, in the same order, regardless of building type or region.
| Phase | Contents |
|---|---|
| Soft costs | Design, engineering, building permit, plan check, impact and connection fees |
| Site work + foundation | Excavation, grading, footings, slab or stem wall, sewer lateral, utility trenching |
| Framing + structure | Lumber package, framing labor, trusses, sheathing |
| Building envelope | Roofing, windows, exterior doors, siding, weather barrier |
| Systems + interior | Electrical, plumbing, HVAC, insulation, drywall, flooring, paint, trim, kitchen, bath |
| Landscaping + hardscape | Paths, stoops, planting, sod, finish grading |
Holding this schema fixed is the single most important methodological decision on the site. It is what makes cross-project comparison possible.
Sources
Primary — regulatory and permit records
- California HCD — Accessory Dwelling Units — the state’s ADU program page
- California HCD ADU Handbook — statewide rules, size limits, fee exemptions. HCD added an Addendum in December 2025 covering changes to State ADU Law effective 1 January 2026
- California Government Code §66311.5 — impact-fee limits for ADUs and JADUs (including the 750 sq ft threshold)
- LA County EPIC-LA permit portal — declared construction valuations, unincorporated county and contract cities
- LA County Building & Safety permit information — permit types, plan check, fee process
- LA County Open Data — EPIC-LA Permit Finder — permit record dataset
- LADBS online building records — City of Los Angeles
- LADBS Permit & Inspection Report — search permits by address
- ZIMAS — City of Los Angeles zoning and parcel constraints
Secondary — cost structure and benchmarks
- NAHB Cost of Construction Survey — phase-level cost shares. Construction costs accounted for 64.4% of the average new home sale price in the 2024 survey, a series high
- NAHB Survey of Construction — per-square-foot price benchmarks by region
- NAHB regulatory cost study, June 2026 — regulation accounts for 26.4% of the final price of a new single-family home
- NAHB research studies index
How a figure is produced
- Define the scenario. Square footage, jurisdiction, construction type, site condition, finish grade — all fixed and stated in the post.
- Anchor to permit records. Pull declared construction valuations for comparable recent builds in the same jurisdiction. This sets the plausible total.
- Allocate across the six phases. Use published phase-level cost shares, adjusted for the specific scenario.
- Sanity-check per square foot. Compare against published per-square-foot benchmarks for the region and construction type — and account for the small-unit premium described below.
- State it as a median, with the variables that move it. Every post lists the factors that push the number up or down and by roughly how much.
Why ~$400/sq ft is not a contradiction of NAHB ~$167/sq ft
Step 4 compares our models to NAHB’s Survey of Construction square-foot prices. For contractor-built (custom) single-family detached homes started in 2024, NAHB’s national median was $166 per square foot; the Pacific division median was $167 per square foot (Eye on Housing, Oct 2025). Those figures exclude improved lot value.
Our Los Angeles County mid-grade detached ADU models land near $400/sq ft (800 sq ft) and $537/sq ft (400 sq ft). That is roughly 2.4× the Pacific custom-home median on the 800 model — and it is intentional, not a failed check.
| What NAHB SOC measures | What our ADU models measure |
|---|---|
| Full-size new single-family detached homes | Accessory units of 400–800 sq ft |
| Economies of scale across a whole house | One kitchen + one bath that barely shrink |
| National / Census division medians | Southern California mid-grade, site-built |
| Contract price ÷ finished floor area | Same six-phase build cost ÷ ADU floor area |
Small-unit premium (why the multiple exists):
- Fixed rooms dominate. A kitchen and a bathroom cost nearly the same at 400 or 1,200 sq ft. On an 800 sq ft ADU they are almost half the budget; on a 2,400 sq ft house they are a much smaller share.
- Fixed site and utility work. Sewer lateral, electrical service run, and foundation mobilization do not scale linearly with floor area.
- Soft costs and fees are front-loaded. Design, plan check, and many agency fees are closer to fixed (or step-function) than to $/sq ft.
- Geography and product. Pacific custom medians mix a wide range of house sizes and markets; our scenarios are small Southern California ADUs with mid-grade finishes.
So the NAHB number is a floor and a scale check, not a target $/sq ft for an ADU. If an ADU model came in near $167/sq ft for a detached SoCal build, we would treat that as suspiciously low relative to permit valuations and phase shares — not as “validated.”
The 750 sq ft impact-fee cliff
California law draws a hard line on many impact fees (not connection or capacity charges):
- An ADU with 750 square feet 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 square footage of the primary dwelling.
See Government Code §66311.5 and the HCD ADU Handbook. School developer fees sit under a separate Education Code framework (with a distinct ~500 sq ft threshold in current law) — always verify with the school district.
That cliff matters for sizing: our 400 model sits below it; the 800 model sits just above it. Soft-cost totals in this series still only diverge by about $4,000 between those two models because design and plan check dominate phase one in Los Angeles County mid-grade scenarios, and many local ADU fee schedules were already compressed. In cities that still levy large proportional impact fees above 750 sq ft, the soft-cost step can be much larger than our LA County medians imply. Details and local verification steps: ADU soft costs.
Known limitations
We would rather state these than have you find them.
- Declared permit valuations understate real cost. Builders declare a valuation for fee purposes, and it is typically lower than the contract price. We use them as a floor and a relative signal, not as the total.
- Medians hide variance. Half of real projects land above the number on the page. Site conditions are the largest single cause.
- Regional data is thin outside Southern California. Every figure on this site is currently anchored to Los Angeles County. Do not transfer it to another state without adjustment.
- Cost data ages fast. Material prices and labor rates move quarterly. Every post carries a “last verified” date for this reason.
- We have not inspected these projects. These are modeled scenarios built from public records, not audited job costs.
- We do not subscribe to proprietary estimating databases. Every source on this page is publicly accessible, deliberately, so that you can open it and check us.
- Impact-fee cliffs are jurisdiction-specific in dollar terms. State law sets the 750 sq ft rule; the dollar step depends on the local fee schedule.
How this content is produced
Research, drafting, and modeling on this site are AI-assisted. Every figure is traced to a named, linked source that you can open and check yourself, and every correction is logged in public.
We disclose this because the alternative — being quiet about it — would contradict the only thing this site is actually selling, which is that you can verify what we tell you.
Corrections policy
See Corrections.