Methodology · Last reviewed August 2026
How the ADU vs. room addition calculator works
Every figure the tool shows you, where it came from, and how the recommendation is decided. Including the numbers we are less confident about and the widely repeated industry claim we chose not to use.
What this tool is, and what it is not
The ADU vs. room addition tool takes five answers — your goal, the space you have, your lot size, your budget and your city — and returns the route most likely to fit, with a cost range, an income or resale figure, and a timeline.
It is a decision aid. It is not a quote, not a code determination and not a substitute for a zoning check on your specific parcel. It works from five inputs, and the things that actually move a construction number by six figures — foundation condition, panel capacity, how far new plumbing has to run, how the roof ties in, whether the lot is hillside — are not among them and cannot be.
We publish this page for two reasons. If you are making a six-figure decision partly on the basis of something we built, you are entitled to check how it works. And a figure whose source we cannot show is a figure we should not be publishing.
Figure by figure
Where each number comes from
Confidence is our own assessment. Where it is medium or low, the tool presents a wider range or adds a caveat rather than implying precision we do not have.
| Figure in the tool | Value | Source | Confidence |
|---|---|---|---|
| Junior ADU cost | $45,000 to $90,000 | Our completed projects | High |
| Garage conversion to ADU | $110,000 to $220,000 | Our completed projects | High |
| Single-room addition | $90,000 to $200,000 | Our completed projects | High |
| Master suite addition | $180,000 to $400,000 | Our completed projects | High |
| Detached office or studio | $60,000 to $150,000 | Our completed projects | High |
| Detached ADU, new build | $150 to $400 per sq ft | Published Los Angeles market ranges | Medium |
| ADU rent, Los Angeles | $2,000 to $3,500 a month | Published Los Angeles rental ranges | Medium |
| Addition recouped at resale | About half of cost | Remodeling, Cost vs. Value 2026 | Medium |
| ADU maximum size | 1,200 sq ft on most lots | Gov. Code §§66310–66342 | High |
| ADU minimum setback | 4 ft side and rear | Gov. Code §§66310–66342 | High |
| Impact fee exemption | Under 750 sq ft | State ADU law | High |
| ADU decision deadline | 60 days, then deemed approved | Gov. Code §66317 | High |
| Completeness determination | 15 business days | SB 543, effective 1 Jan 2026 | High |
| Standard plan review time | 4–6 weeks down to about a day | LADBS ADU Standard Plan Program | Medium |
How the recommendation is decided
The logic is deliberately simple, because a more elaborate model would imply a precision that five questions cannot support.
Your goal does most of the work. Rental income and housing a relative both require a separate dwelling unit, because a room addition cannot be let or occupied independently — that is a legal constraint, not a preference, and it settles those cases immediately. Wanting more room for your existing household points the other way just as firmly, since an addition delivers connected space without the separate-utility and access requirements a dwelling unit carries.
Your available space then chooses the variant. An existing garage makes conversion the cheapest route to an ADU by a wide margin, because the slab, walls and roof are already built and paid for. A tight lot pushes a dwelling unit inside the existing footprint as a junior ADU, where setbacks and lot coverage are not the binding constraint.
Your lot size and budget act as filters rather than drivers. A lot under 5,000 square feet is treated as tight regardless of what you selected, because setbacks and lot coverage usually bind before anything else does. A budget under $100,000 rules out new detached construction, so the tool routes to a conversion instead of showing you something you cannot fund.
Your city does not change the recommendation. It changes what we tell you about permitting, because LADBS only has jurisdiction inside City of Los Angeles limits and the other cities we serve run their own departments with their own fee schedules and queues.
The tool always shows a second option alongside the first. That is deliberate. A single confident answer to a question this consequential would be overselling what five inputs can determine.
The number we decided not to use
You will repeatedly see the claim that an ADU returns 125 to 150 per cent of its construction cost in added property value. We looked at where it comes from, and it traces almost entirely to companies whose business is selling ADUs. We could not find independent research supporting it at that level, and it sits badly against the resale data for additions, which national figures put at roughly half of cost recouped.
The two are not measuring quite the same thing — the argument for ADUs is that an income-producing unit gets valued on its income rather than its square footage, which is not unreasonable in principle. But appraisal practice for accessory units in California is still inconsistent, and how yours is treated depends heavily on whether comparable sales exist nearby.
So the tool does not use it. For accessory dwelling units it reports rental income, which is measurable and which you receive whether or not an appraiser agrees. For additions it reports the resale figure with its source attached. Presenting a sales claim as though it were research would undermine the point of publishing a methodology at all.
Known limits
Things this tool does not currently account for, stated plainly so you can judge how much weight to put on it.
- Hillside lots. The Baseline Hillside Ordinance substantially changes what is buildable in hillside areas, and several neighbourhoods we serve are hillside-heavy. The tool flags this where relevant but does not model it.
- Your actual zoning. Recommendations assume a standard single-family lot. Overlay zones, specific plans, historic preservation zones and coastal areas all impose additional constraints that only a parcel-level check will reveal.
- Permit and plan check fees. Excluded from all cost ranges. They vary by jurisdiction and are passed through at cost on our estimates. A dedicated permit estimator is in progress.
- Site conditions. Foundation type, panel capacity, plumbing runs, roof structure and soil. Collectively these move a number more than any input the tool asks for.
- Rent variation within Los Angeles. The rental range is a market-wide figure. Neighbourhood, unit size and whether there is parking move it considerably inside that range.
- Legislative change. California ADU law has changed materially in each of the last several years. This page carries a review date for that reason.
Sources
What we relied on
Statutory citations reflect the post-SB 477 numbering. Page last reviewed August 2026, and reviewed again whenever the underlying law or fee schedules change.
- California Government Code sections 66310 to 66342. State ADU and junior ADU law as reorganised by SB 477. Section 66317 governs the 60-day approval deadline, deemed approval and ministerial review. Source for size limits, setbacks and the impact fee exemption.
- California Department of Housing and Community Development — Accessory Dwelling Units. The state agency's own guidance and handbook on ADU standards, and statewide permitting data.
- LADBS ADU Standard Plan Program. Source for the pre-approved plan route and the reduction in plan check time. Also the basis for our statement that LADBS jurisdiction stops at City of Los Angeles limits.
- Remodeling — Cost vs. Value Report 2026. Source for the proportion of an addition's cost recouped at resale, nationally and for the Pacific region.
- Harvard Joint Center for Housing Studies. Remodeling market context, including the Leading Indicator of Remodeling Activity and the Center's published research on accessory dwelling units.
- Terner Center for Housing Innovation, UC Berkeley. Research on California ADU production and the effect of state legislation on it.
- AB 1033 (2023). Permits cities to allow separate condominium sale of ADUs. Los Angeles is among the jurisdictions where this is active.
- SB 543 (2025), effective 1 January 2026. Fifteen business day completeness determination.
- Redondo Beach and others v. Bonta, and the subsequent Court of Appeal decision of 12 November 2025. Procedural history behind our statement that SB 9 currently stands statewide but is not finally settled.
- Our own completed Los Angeles projects. Every cost range marked high confidence. We are a licensed California contractor, CSLB #886079, and these are our own figures rather than a national average adjusted for the city.
Found a problem?
Tell us and we will fix it
If a figure here is out of date, a citation has moved, or you think our reasoning is wrong, we would genuinely like to know. Corrections make the tool better and we would rather hear it from you than keep publishing something inaccurate.
Call (626) 726-3989 or email [email protected].
If what you actually want is a real number for your house, that is the estimate conversation instead — or go back to the ADU vs. room addition tool and start there.