Research study
How big are the ADUs people actually build? The rule picks the number
Every competitor page says “typically 600 to 800 square feet” and none of them says where that came from. Here is the measured distribution, and the two statutory lines it is piled against.
Written by HyreADU Research Desk Primary-source research and data analysis
Audited by HyreADU Research Desk Statutory citation and distribution audit
The finding
On 8,536 new detached ADU building permits issued by the City of Los Angeles between 1 January 2020 and 2026-08-30, the median permitted floor area is 920 square feet, with the middle half between 670 and 1,194 square feet.
The median is the least interesting number on this page, because the distribution is not shaped like a distribution of household needs. It is bimodal, and both modes sit exactly on a number written into the California Government Code. The larger mode is at the cap. Government Code section 66314(a)(5) caps a detached ADU at 1,200 square feet. 26.0% of these permits declare a floor area between 1,100 and 1,200 square feet, and 635 declare exactly 1,200, against 79 in the bin immediately above.
The smaller mode is at the floor. Section 66321 requires a local ordinance not to prevent an ADU of at least 800 square feet of interior livable space with four-foot side and rear setbacks. 254 permits declare exactly 800 square feet, and the 750–800 bin is the second-largest in the whole distribution.
A third line is visible but weaker. Section 66311.5, the fee section, bars any impact fee on an ADU of 750 square feet of interior livable space or less; 32.5% of these permits fall at or under that.
The junior ADU obeys its own number: 76% of JADU-flagged permits declare 500 square feet or less, the statutory ceiling on a junior unit.
Los Angeles is not California and it is not the United States. A permitted floor area is what an applicant declared and a department recorded; it is not a measurement we made of a finished building.
Read this first
- One city, and one permit type inside it
The headline frame is new-construction building permits in the City of Los Angeles that the department flagged as adding exactly one ADU.
That is deliberate: on a new-construction permit the recorded floor area is a good proxy for the finished unit, and on an alteration or addition permit it is frequently the scope of work instead.
Conversions and additions appear on this page in their own sections, labeled, and they are not mixed into the headline.
- This is a declared floor area, not a measured one
The
square_footagefield is what the applicant put on the application and what LADBS recorded. HyreADU did not measure any building. Where an applicant rounded to 1,200 because that is the number the ordinance allows, the record says 1,200 — and that is exactly the behavior this page is reporting. - A clustering is evidence of a binding constraint, not of a bad decision
A pile-up just under a legal ceiling means the ceiling is binding for a large number of households — that they would have built larger if the rule allowed it, or that they sized to the rule rather than to the brief.
It does not tell you the rule is wrong, and it does not tell you a 1,200-square-foot unit is the right unit for any particular lot.
- We publish the trimming, we do not hide it
9,374 permits are in the new-construction frame. 838 carry a recorded floor area below 100 or above 3,000 square feet and are excluded from the distribution as implausible for a single accessory unit — a whole-parcel figure, a keying error, or a permit whose square footage describes something other than the ADU.
That is 8.9% of the frame, and dropping them moves the median; the untrimmed figures are published in the frame table below.
- Statutory descriptions, not legal advice
The sections quoted here were retrieved from the Legislative Counsel’s own service on 5 September 2026 and are quoted rather than paraphrased.
Whether any of them applies to your lot, and how your city’s ordinance interacts with them, is a legal question this page does not answer.
The chapter has been renumbered twice since March 2024 and most published guidance still cites repealed sections.
The distribution, and the two lines it is piled against
A distribution of things people freely chose would be smooth. This one is not. It rises through the 400s, sags through the 850–1,050 band, and then climbs into a spike at 1,150–1,200 that is the tallest bar on the chart — 1,066 permits in a single 50-square-foot bin, against 79 in the next bin up.
Fact. Government Code section 66314(a)(5) reads, in the text retrieved on 2026-09-05: “The total floor area for a detached accessory dwelling unit shall not exceed 1,200 square feet.” Section 66321 separately provides that a local ordinance may not fail to permit “an accessory dwelling unit with at least 800 square feet of interior livable space and with four-foot side and rear yard setbacks … to be constructed in compliance with all other local development standards,” and sets 850 square feet, or 1,000 square feet for a unit with more than one bedroom, as maximums a local agency is allowed to impose.
HYRE analysis. The two tallest features of this distribution sit on two of those numbers. 635 permits declare exactly 1,200 square feet — more than twice the frequency of any other single value. 254 declare exactly 800.
Both are followed by a cliff: the count above 1,200 collapses by a factor of about 13, and the count immediately above 800 falls by a factor of about 2.7. A statistical distribution does not produce that shape. A rule does.
The weaker third line is the money one. Section 66311.5 bars any impact fee on an ADU with “750 square feet of interior livable space or less”, and requires any impact fee above that to be charged proportionately to the primary dwelling. 32.5% of these permits are at or below 750 square feet.
There is a visible local mode in the 700–800 band, but the pile is above the fee line rather than below it, which is what you would expect if the 800-square-foot constructability floor is the stronger magnet of the two.
We report both and we do not pretend to have separated their effects.
What this is not. It is not evidence that the 1,200-square-foot cap is too low, or that it is right.
It is evidence that it binds — that for roughly a quarter of new detached ADUs in Los Angeles, the size of the unit is a legal number rather than a household’s number.
What follows from that is a policy argument, and it is not ours to make on a research page.
The histogram as a table
Every bar in the chart above, with its count and its share, so the chart is reproducible from this page alone. 50 square foot bins; the bin label is its lower bound and the bin is closed at the bottom and open at the top.
| Floor area (sq ft) | Permits | Share of frame | What sits here |
|---|---|---|---|
| 150–199 | 11 | 0.1% | |
| 200–249 | 34 | 0.4% | |
| 250–299 | 76 | 0.9% | |
| 300–349 | 133 | 1.6% | |
| 350–399 | 206 | 2.4% | |
| 400–449 | 287 | 3.4% | |
| 450–499 | 434 | 5.1% | The efficiency-unit end of the range |
| 500–549 | 223 | 2.6% | § 66313 caps a junior ADU at 500 sq ft of interior livable space |
| 550–599 | 289 | 3.4% | |
| 600–649 | 321 | 3.8% | |
| 650–699 | 310 | 3.6% | |
| 700–749 | 417 | 4.9% | |
| 750–799 | 673 | 7.9% | Second mode. Just above the § 66311.5 impact-fee line and at the § 66321 800 sq ft constructability floor |
| 800–849 | 507 | 5.9% | Contains the 254 permits declaring exactly 800 sq ft |
| 850–899 | 253 | 3.0% | § 66321 names 850 sq ft as a maximum a local agency may impose on a one-bedroom unit |
| 900–949 | 234 | 2.7% | |
| 950–999 | 232 | 2.7% | |
| 1,000–1,049 | 290 | 3.4% | § 66321 names 1,000 sq ft as the equivalent for a unit with more than one bedroom |
| 1,050–1,099 | 295 | 3.5% | |
| 1,100–1,149 | 541 | 6.3% | |
| 1,150–1,199 | 1,066 | 12.5% | Largest bin. Contains the 635 permits at exactly 1,200 sq ft minus those at 1,200 itself; the run 1,195–1,200 alone holds 1,061 |
| 1,200–1,249 | 728 | 8.5% | The § 66314(a)(5) detached cap. Everything from here up is a small minority |
| 1,250–1,299 | 79 | 0.9% | The cliff |
| 1,300–1,349 | 54 | 0.6% | |
| 1,350–1,399 | 38 | 0.4% | |
| 1,400–1,449 | 39 | 0.5% | |
| 1,450–1,499 | 48 | 0.6% | |
| 1,500–1,549 | 40 | 0.5% | |
| 1,550–1,599 | 43 | 0.5% | |
| 1,600–1,649 | 51 | 0.6% |
n = 8,536 new detached ADU permits with a recorded floor area between 100 and 3,000 sq ft. Bins below 150 and above 1,650 sq ft each hold under 1% and are omitted for length; they are in the source resource. HYRE calculation on LADBS pi9x-tg5x, retrieved 2026-09-05.
The statutory numbers, and how much of the distribution obeys each
Each row pairs a number written into the Government Code with the share of the measured distribution that sits on the relevant side of it. The statutory text is quoted from the Legislative Counsel’s service, not paraphrased.
| Statutory number | What the section actually says | Section (current numbering) | What the LA distribution does |
|---|---|---|---|
| 1,200 sq ft | “The total floor area for a detached accessory dwelling unit shall not exceed 1,200 square feet.” | Gov. Code § 66314(a)(5). Amended by Stats. 2025, Ch. 67 (AB 1170), effective 1 Jan 2026. Retrieved 2026-09-05. | 26.0% of permits fall in the 1,100–1,200 band; 1,061 declare between 1,195 and 1,200; 635 declare exactly 1,200. Only 12.5% exceed it. |
| 800 sq ft | A local ordinance is non-compliant if it “does not permit an accessory dwelling unit with at least 800 square feet of interior livable space and with four-foot side and rear yard setbacks to be constructed in compliance with all other local development standards.” | Gov. Code § 66321. Amended by Stats. 2025, Ch. 520, Sec. 5 (SB 543), effective 1 Jan 2026. Retrieved 2026-09-05. | 254 permits declare exactly 800 sq ft — the second most common single value in the distribution. The 750–800 bin holds 673, the 800–850 bin 507, and the 850–900 bin 253. |
| 850 / 1,000 sq ft | “Eight hundred fifty square feet of interior livable space”, or “[o]ne thousand square feet of interior livable space for an accessory dwelling unit that provides more than one bedroom”, as maximums a local agency is permitted to impose. | Gov. Code § 66321. Retrieved 2026-09-05. | This is the trough, not a mode: the 850–1,050 band is the flattest part of the distribution. Los Angeles evidently does not cap at 850 or 1,000, which is consistent with the 1,200 sq ft mode being the binding one here. |
| 750 sq ft | “A local agency, special district, or water corporation shall not impose any impact fee upon the development of an accessory dwelling unit that has 750 square feet of interior livable space or less.” Above that, impact fees “shall be charged proportionately in relation to the square footage of the primary dwelling unit.” | Gov. Code § 66311.5. Added by renumbering § 66324 by Stats. 2025, Ch. 520, Sec. 7 (SB 543), effective 1 Jan 2026. Retrieved 2026-09-05. | 32.5% of permits are at or below 750 sq ft. The mode near this line sits above it, at 800, not below it — so on this evidence the fee threshold is the weaker of the two magnets in this city. |
| 500 sq ft | A junior accessory dwelling unit is “no more than 500 square feet of interior livable space … contained entirely within a single-family residence”. | Gov. Code § 66313(d). Amended by Stats. 2025, Ch. 520 (SB 543), operative 1 Jan 2026. Retrieved 2026-09-05. | 76% of the 2,238 JADU-flagged permits with a usable floor area are at or below 500 sq ft; the JADU median is 409 sq ft. The units that exceed it are almost certainly permits where the recorded square footage describes the house, not the junior unit. |
Section numbers are the current ones. The ADU chapter was moved out of Government Code § 65852.2 by SB 477 (Stats. 2024, Ch. 7) with effect from 25 March 2024, and renumbered again inside Chapter 13 by SB 543 with effect from 1 January 2026.
A source citing § 65852.2 is two recodifications stale; a source citing § 66324 is one.
Size by area planning commission
The same new-construction frame, split on the department’s own area-planning-commission field. The Valley builds bigger; the Westside builds smaller. The 1,100–1,200 share is the clearest single indicator of how hard the cap is binding in each place.
| Area planning commission | Permits | Median (sq ft) | 25th–75th percentile | At or under 750 sq ft | In the 1,100–1,200 band |
|---|---|---|---|---|---|
| North Valley | 2,422 | 1,104 | 799 – 1,200 | 19.9% | 37.3% |
| South Valley | 2,456 | 953 | 727 – 1,196 | 28.5% | 28.9% |
| South Los Angeles | 1,355 | 878 | 622 – 1,163 | 37.3% | 18.3% |
| Harbor | 243 | 858 | 701 – 1,189 | 32.1% | 18.1% |
| Central | 400 | 798 | 532 – 1,178 | 45.3% | 17.0% |
| East Los Angeles | 801 | 784 | 559 – 1,143 | 46.1% | 19.5% |
| West Los Angeles | 859 | 727 | 480 – 1,020 | 53.5% | 10.6% |
New-construction ADU permits with a floor area between 100 and 3,000 sq ft, by LADBS area planning commission. Commissions with fewer than 100 such permits are omitted. HYRE calculation on pi9x-tg5x, retrieved 2026-09-05.
Read this as a description of where big ADUs get permitted, not as a lot-level rule. Median lot size, prevailing zone and the age of the housing stock all differ across these areas, and this table separates none of them. The geography itself is the subject of a separate study.
The other two lanes, kept separate
Garage and structure conversions
Median recorded floor area on a permit describing a garage conversion is 480 square feet, with the middle half between 373 and 745.
That is materially smaller than the new-build frame, and the reason is not preference: a conversion that stays inside the existing exterior dimensions keeps its setback exemption, and a two-car garage footprint is what it is.
Nearly half of all Los Angeles ADU permits are conversions. We measure that share, its declared valuations and its completion behavior on the garage-conversion study, and we do not fold it into the headline here because the square-footage field means something different on an alteration permit.
Junior ADUs
3,778 permits in the wider frame are flagged by the department as junior ADUs. Of those with a usable floor area, the median is 409 square feet and 76% are at or under 500 — the statutory ceiling in Government Code § 66313(d).
A junior ADU is not a small ADU. It is a different unit with a different statutory definition, a different owner-occupancy rule, and a requirement to sit entirely within the single-family residence.
Its size distribution obeys its own number, which is exactly what you would expect and is the cleanest confirmation on this page that these piles are statutory rather than accidental.
Method
Reproducible from one named public resource and five sections of the Government Code.
- 1 Download the whole permit resource
All 409,619 rows of LADBS Socrata resource
pi9x-tg5x, Building Permits Issued from 2020 to Present, downloaded on 2026-09-05. The department’s ownrefresh_timefield runs to 2026-08-30. - 2 Use the department’s ADU flag
A row enters the wide frame if
adu_changedis greater than zero orjunior_aduis 1 — LADBS fields, not ours — and if it is not a supplemental permit. That is 39,279 permits issued from 2020. - 3 Narrow the headline frame to new construction adding exactly one unit
permit_type = 'Bldg-New'andadu_changed = 1. 9,374 permits. This is the only cut on which the recorded square footage is a reliable proxy for the ADU itself, and restricting to it is the single most important methodological choice on this page. - 4 Trim implausible floor areas, and say how many
Values below 100 or above 3,000 square feet are dropped: 838 permits, 8.9% of the frame. Untrimmed, the median of the same frame is 1,001 square feet and the mean is 1,358; trimmed they are 920 and 975. Both are published so a reader can see what the trim did.
- 5 Read the statute, do not summarize a summary
Sections 66311.5, 66313, 66314 and 66321 were retrieved individually from the Legislative Counsel’s
codes_displaySectionservice and quoted verbatim. Section numbers are the post-SB 543 ones operative from 1 January 2026, which is why they do not match most published ADU guidance. - 6 Report percentiles, never a single "typical" number
A bimodal distribution has no typical value, and the mean of one is actively misleading. The median is published because readers expect it; the bins, the quartiles and the modes are published because they are what the distribution actually is.
What a homeowner should take from this
Not that you should build 1,200 square feet. The pile at the cap tells you what other people’s lots and other people’s budgets allowed; it says nothing about yours.
What is worth taking is the shape of the decision. There are two size thresholds on the way up that change something concrete: 750 square feet, above which impact fees become chargeable proportionately under § 66311.5, and the point at which your city’s own standards — setback, lot coverage, height — start binding before the state cap does. Between those, the constraint is money and the yard.
And a size figure is not a cost figure. Our permit-valuation study shows why the dollars attached to these permits are not construction costs, and the cost study sets out the anchors that are defensible.
Terms, used precisely
- Permitted floor area
- The LADBS
square_footagevalue recorded against the permit. A declared figure, not a HYRE measurement, and not necessarily identical to “interior livable space” as the Government Code uses that phrase. - Interior livable space
- The measure the statute uses for the 750-square-foot fee threshold and the 500-square-foot junior-ADU ceiling. It is not guaranteed to be the same quantity as a permit’s recorded square footage, and where the two differ our shares are approximations. We say so rather than assuming they match.
- Bimodal
- A distribution with two separate peaks rather than one. Here the peaks are at roughly 800 and roughly 1,200 square feet, and a single “average ADU size” drawn from them would describe a unit that is unusually rare.
- Binding constraint
- A limit that is actually changing behavior, evidenced by a pile-up immediately beneath it and a collapse immediately above it. Both signatures are present at 1,200 square feet in this data.
- New-construction frame
- The 9,374 permits with
permit_type = 'Bldg-New'andadu_changed = 1. The headline distribution on this page and nothing wider.
Questions
What is the average ADU size?
How big can an ADU be in California?
Why do so many ADU permits say exactly 1,200 square feet?
Is there a size below which I do not pay impact fees?
Why did you use section 66311.5 when every other page says 66324?
How big is a typical garage conversion?
How big is a junior ADU?
Does this hold outside Los Angeles?
Written and audited by
HyreADU Research Desk
Primary-source research, data analysis and fact checking
We are a research desk, not a builder. We read the permit extract, the statute, the HCD return or the fee schedule ourselves, and publish each figure with its source and retrieval date.
Where a number cannot be traced to a primary source, we leave it out and say what we could not verify. Our store-based claims cover California only.
- CA
- the only state this desk will make store-based claims about
- 5
- jurisdictions with extracted ADU permit evidence
- 735
- CSLB-verified companies in the California store
- 0
- national claims from a one-state store
How this desk works
- Primary sources only. Permit counts come from the city or county that issued the permit. Production counts come from HCD’s Annual Progress Report. Rents come from HUD or the Census. We do not cite an article that cites a source; we download the source and compute the figure ourselves.
- This is a California site. The company store is 734 California firms and one New Mexico firm. Permit evidence exists for five named jurisdictions: Los Angeles, San Francisco, Sacramento, San José and unincorporated Marin. A number from that store is titled to those places, never to the United States.
- A permit is not a completion, and a license is not an ADU grade. California licenses no ADU classification. Being named on an ADU permit is evidence of engagement in that jurisdiction, not of quality, completion, or work anywhere else. Owner-builder permits are excluded from contractor counts.
- Calculation is labeled as calculation. Figures we derive are never presented as something HCD, HUD, the Census or a city published. Terner Center research is cited as Terner’s, never restated as ours.
- We do not design, permit or build ADUs, and we take no payment for placement, ranking or a favorable mention. Pages that look like rankings are not: they publish public-record counts and let the reader decide.
- Nothing here is legal, tax or financial advice. Zoning, underwriting and appraisal practice vary by jurisdiction, lender and appraiser. The useful next step on a specific lot is the planning counter and a licensed professional.
Data as of LADBS Socrata resource pi9x-tg5x (409,619 rows) downloaded and analyzed 2026-09-05; Government Code §§ 66311.5, 66313, 66314 and 66321 retrieved from leginfo.legislature.ca.gov 2026-09-05. Authorship on this site is organizational: the analysis belongs to the desk rather than to a named individual, and we do not publish credentials we do not hold.
Our editorial policy sets out how we source, date and correct what we publish.
Sources & retrieval dates
- City of Los Angeles Department of Building and Safety — Building Permits Issued from 2020 to Present (Socrata resource pi9x-tg5x) , Full resource download, 409,619 rows, retrieved 2026-09-05. Fields used: permit_nbr, permit_type, issue_date, square_footage, apc, adu_changed, junior_adu, refresh_time. ADU frame from the departmental flags; supplementals excluded by permit-number segment; headline frame restricted to permit_type = 'Bldg-New' with adu_changed = 1. Retrieved 2026-09-05.
- California Government Code § 66314 — maximum standards for an ADU, including the 1,200 square foot detached cap , Quoted verbatim on this page: “The total floor area for a detached accessory dwelling unit shall not exceed 1,200 square feet” at subdivision (a)(5), and “the total floor area of an attached accessory dwelling unit shall not exceed 50 percent of the existing primary dwelling” at (a)(4). Amendment history line as retrieved: “Amended by Stats. 2025, Ch. 67, Sec. 109. (AB 1170) Effective January 1, 2026.” Retrieved 2026-09-05.
- California Government Code § 66321 — size standards a local agency may and may not impose , Quoted verbatim: the 800 square foot constructability requirement (“does not permit an accessory dwelling unit with at least 800 square feet of interior livable space and with four-foot side and rear yard setbacks to be constructed in compliance with all other local development standards”), and the 850 / 1,000 square foot maximums. Amendment history as retrieved: “Amended by Stats. 2025, Ch. 520, Sec. 5. (SB 543) Effective January 1, 2026.” Retrieved 2026-09-05.
- California Government Code § 66311.5 — impact fees and the 750 square foot threshold , Quoted verbatim: “A local agency, special district, or water corporation shall not impose any impact fee upon the development of an accessory dwelling unit that has 750 square feet of interior livable space or less or a junior accessory dwelling unit that has 500 square feet of interior livable space or less,” and the proportional-charging sentence. History as retrieved: “Added by renumbering Section 66324 by Stats. 2025, Ch. 520, Sec. 7. (SB 543) Effective January 1, 2026.” Retrieved 2026-09-05.
- California Government Code § 66313 — definitions, including the 500 square foot junior ADU , The junior ADU definition used for the JADU section of this page: “no more than 500 square feet of interior livable space … contained entirely within a single-family residence”. Retrieved 2026-09-05.
- SB 543 (McNerney, 2025) — Accessory dwelling units. Stats. 2025, Ch. 520 , The renumbering that moved § 66324 to § 66311.5, § 66327 to § 66313.5 and § 66332 to § 66311.7 with effect from 1 January 2026, and amended § 66321. Cited so that a reader can check why the section numbers on this page differ from almost every other published ADU page. Retrieved 2026-09-05.
- City of Los Angeles open data portal — dataset landing page for pi9x-tg5x , Publisher description and column definitions, used to confirm that adu_changed, junior_adu and square_footage are departmental fields rather than derived ones. Retrieved 2026-09-05.
The cap that binds on your lot is probably not the state one
The 1,200-square-foot ceiling is the last constraint most projects hit. Setback, lot coverage, height and the shape of the yard usually bind first. The size-envelope tool asks those questions in order, and it will never tell you that you can build.
HyreADU does not design, permit or build accessory dwelling units, and takes no payment for placement or a favorable mention. This page is informational and is not legal advice.
Statutory text is quoted as retrieved from the California Legislative Counsel’s service on 5 September 2026; the ADU chapter has been renumbered twice since March 2024 and your city’s ordinance may not have caught up.
Whether any threshold on this page applies to your property is a legal and planning question for the permitting agency and, where money is at stake, a licensed professional.
Every count, share and percentile is the City of Los Angeles only, computed from a named public permit extract; it is not a California or national statistic.
Permitted floor area is a declared value recorded by the department, not a measurement of a finished building.