Do ADUs add value?

The question everybody asks first

“Does an ADU add value?” is the first question a homeowner asks, and it is the question the category has answered with a percentage.

Builder sites, realtor roundups and financing landers repeat a band — most often 20 to 30 percent of the host house, sometimes 25 to 35 percent in the Bay Area — and they often attribute it, vaguely, to UC Berkeley or the Terner Center.

We went looking for the study. We did not find it. Terner Center and Center for Community Innovation publications that we retrieved on 5 September 2026 measure construction cost, who builds, how long permitting takes, what the units rent for, and who lives in them.

They do not publish a sale-price premium for ADU properties. The GatherADU post that attributes a 25–35 percent Bay Area premium and a 20–30 percent Los Angeles County premium to Terner names no paper, no table and no method. We will not repeat the figure as a fact.

HyreADU analysis: a percentage without a geography, a year, a comparable set and a method is marketing copy. The honest literature is thinner than the copy, and thinner is the finding.

What can be assembled from primary sources is the assignment the appraiser is actually given, the income the GSEs will and will not count, the way a California assessor will add the unit to the roll, and the two academic papers that tried to measure a differential at all.

The Los Angeles study, and what it will not carry

Jan K. Brueckner and Sarah Thomaz, “ADUs in Los Angeles: Where are they located and by how much do they raise property value?”, Real Estate Economics 52, no. 3 (2024): 885–907, is the paper the category actually has.

Using City of Los Angeles tax-assessor parcel data for 2013–2019, they estimate locational determinants of ADU presence and then regress assessed value on that presence.

The assessed-value regressions “show that ADU presence raises a parcel’s assessed value and selling price by 7%–9%, while also accurately capturing the unusual rules for property assessments under California’s Proposition 13.” For the entire sample, including parcels that have and have not just sold, “an ADU raises assessed value by 7%–8%.” For just-sold properties — “where the results give a more-reliable picture of an ADU’s sales-price impact” — the effect is “almost 9%.” Column 1 of the all-property specification is 7.86 percent.

The authors are careful, and we should be too. These are ordinary-least-squares estimates that treat ADU presence as exogenous.

An alternate instrumental-variable approach, using a dummy for assessment years after the 2016 ADU-enabling bills (SB 1069 and AB 2299), was “unsuccessful.” For properties that have not recently sold, “the increment to assessed value may instead rely on the assessor’s estimate of the construction cost of the ADU, which may not correspond to the incremental value upon sale.” That is the Prop 13 machine showing through the regression, not a market clearing price.

HyreADU analysis: 7–9 percent in one city, on a 2013–2019 panel that predates the post-2020 ADU boom, with an identification strategy the authors themselves could not make work, is a real number and a modest one. It is not 20–30 percent.

It is not California. It is not your lot. A reader who wants a statewide or national percentage does not have one, and inventing the interpolation would be the same sin as the blogs.

What the Appraisal Institute actually published

The professional body has two documents that matter here, and neither of them is a percentage.

Sandra K. Adomatis, SRA, “Valuation of Accessory Dwelling Units,” The Appraisal Journal 89, no. 4 (Fall 2021): 248–, is the peer-reviewed practice paper.

Its opening claim is historical and operational: “Prior to the 2020 Fannie Mae Selling Guide update, it was next to impossible for appraisers to meet federal Agency appraisal guidelines, because three sales of comparable ADU properties were extremely difficult to find in the local market.” Consequently, she writes, ADU owners were pushed toward private lenders and credit unions that did not sell to the GSEs.

The 2020 update — aged sales accepted, listings as supplemental exhibits — is the flexibility the market had been missing, not a finding that comps had suddenly appeared.

The data-search section is the part that has not dated. “All MLSs do not have searchable fields to identify ADU properties. Public records also may not separate the ADU living area from the main structure.” Adomatis lists more than twenty search terms (granny flat, casita, ohana unit, carriage house, in-law suite, JADU, Next Gen, backyard cottage) because that is what the work actually looks like.

She walks three properties where the MLS description and the assessor record disagree about whether the second unit exists, how big it is, and whether its square footage has been folded into the primary GLA — a material error if the appraiser does not catch it.

Competency, in this assignment, is the ability to find the unit in the data, not the ability to apply a canned adjustment.

Exhibit 1 of that article compares Fannie Mae, Freddie Mac, VA and HUD/FHA ADU rules as of 2021.

The durable rows: all four keep ADU area out of primary GLA; Fannie and Freddie did not then (and Fannie still does not, in the live B2-3-04 text) require the ADU to be legal to finance, while VA and FHA did; dated sales are accepted by all four; Fannie and Freddie require a separate means of ingress.

The rental-income rows in that 2021 exhibit are no longer current for Fannie Mae — see SEL-2025-08 below — and we do not treat a 2021 table as the 2026 rule.

Separately, the Appraisal Institute sells a seminar, Valuation Overview of Accessory Dwelling Units, effective 21 March 2022. It is education material, not a measurement.

We name it so a reader looking for “The Appraisal of ADUs” can find the real document rather than a blog about it.

The Institute’s Dictionary of Real Estate Appraisal, 6th ed. (2015), defines an ADU as “a small, self-contained dwelling, typically with its own entrance, cooking, and bathing facilities, that shares the site of a larger, single-unit dwelling.” That is the professional definition.

Agency definitions, quoted in the next section, are the ones that decide a loan.

Fannie Mae: the assignment, not a factor

The live Selling Guide, PDF dated 2 September 2026, is the rule. Blogs about the rule are not.

B2-3-04, Special Property Eligibility Considerations (8 October 2025). “An ADU is typically an additional living area independent of the primary dwelling that may have been added to, created within, or detached from a primary one-unit dwelling. The ADU must provide for living, sleeping, cooking, and bathroom facilities and be on the same parcel as the primary one-unit dwelling.” Classification requirements in the current HTML text: only one ADU on the parcel of a primary one-unit dwelling; ADUs are not permitted with a two- to four-unit dwelling; the ADU must be subordinate in size; it must have separate means of ingress/egress, kitchen, sleeping area, bathing area and bathroom facilities; a unit that can only be accessed through the primary dwelling, or that is open to it with no expectation of privacy, is not an ADU.

The kitchen minimum is cabinets, a countertop, a sink with running water, and a stove or stove hookup — “hotplates, microwaves, or toaster ovens are not acceptable stove substitutes.” An independent second kitchen by itself is not an ADU; removing the stove does not de-classify one.

Whether the property is a one-unit with an ADU or a two- to four-unit property “will be based on the characteristics of the property, which may include, but are not limited to, the existence of separate utility meter(s), a unique postal address, and whether the unit can be legally rented.” The appraiser decides that in Highest and Best Use.

Zoning: an ADU is “always considered legal if it is allowed under the current zoning code.” Legal nonconforming (predating the ordinance) is contemplated.

An ADU that is not allowed under zoning at all may still be eligible if the lender confirms insurance will not be jeopardised and the B4-1.3-05 appraisal conditions are met.

B4-1.3-05, Improvements Section of the Appraisal Report (4 June 2025). This is the valuation rule.

ADU living area “should not be included with the finished above-grade square footage calculation of the primary dwelling. It should be reported and adjusted for on a separate line in the grid,” unless the ADU is contained within or part of the primary dwelling with interior access and above grade.

A standalone structure that does not meet ADU minimums is treated as any other ancillary structure “and included as a separate line item in the sales comparison approach then adjusted based on its contributory value.” When there is an ADU, “the appraisal report must include a description of the ADU and analysis of any effect it has on the value or marketability of the subject property.” And then the sentence that is the whole finding of this page: “An aged settled sale will qualify as a comparable, and an active listing or under contract sale will qualify as a supplemental exhibit to show marketability.” For an illegal ADU, the report must demonstrate typicality “through an analysis of at least two comparable sales with the same non-compliant zoning use,” with a minimum of three settled sales in the report; aged sales with the same non-compliant use are acceptable if recent sales are not available.

B4-1.3-10, Cost and Income Approach to Value (4 June 2025 / 2 September 2026 in the UAD 3.6 supplement). Appraisals that rely solely on the cost approach as an indicator of market value are not acceptable (manufactured homes excepted).

Appraisals that rely solely on the income approach are not acceptable either.

The income approach “may be appropriate in neighborhoods that consist of one-unit properties when there is a substantial rental market” and “may not be appropriate in areas that consist mostly of owner-occupied properties because adequate rental data does not exist.” That is Fannie Mae telling the appraiser not to capitalise a rent into a value when the neighborhood does not trade that way — which is most single-family neighborhoods.

B3-3.8-02, Rental Income from the Subject Property (2 September 2026), implementing SEL-2025-08 (8 October 2025). This is underwriting, not value, and the two are constantly confused.

Fannie Mae now allows rental income from an ADU toward qualifying income, with three hard limits: the property must be a one-unit principal residence; purchase or limited cash-out refinance only; rental income from one existing ADU only, even if multiple ADUs exist; “the qualifying rental income amount from the ADU is limited to 30% of the total qualifying income.” Desktop Underwriter version 12.1 was scheduled for Q1 2026 to include the eligibility; lenders could implement immediately for manual underwriting.

Gross rent is haircut 25 percent for vacancy and loss on a purchase. A 30 percent cap on qualifying income is not a 30 percent value-add. It is a credit-risk governor.

UAD 3.6 expansion, announced December 2025, effective 31 March 2026, UAD 3.6 appraisals only (Fannie Mae Appraiser Update, March 2026). The Selling Guide B2-3-04 HTML retrieved on 5 September 2026 still states the one-ADU / no-ADU-on-2-to-4-unit rule.

In parallel, Fannie Mae has expanded ADU eligibility for loans with UAD 3.6 appraisals: one-unit properties with up to three ADUs; two- and three-unit properties with ADUs provided dwelling units plus ADUs do not exceed four; limited manufactured-housing ADU configurations.

Appraisers asked for an ADU market-rent estimate “should be based on true ADU rentals when available. When ADU rental comparables are scarce, appropriate substitute rentals may be used.” Scarce, again, is the word in the primary source.

Freddie Mac: one comparable, when you can find one

Freddie Mac’s ADU property rule lives in Guide section 5601.2; the rental-income overlay lives in Chapter 5306. The February 2026 ADU fact sheet is the seller-facing digest of both. We quote the Guide and the fact sheet, not a webinar about them.

Property eligibility (5601.2 / fact sheet). Freddie Mac will purchase a mortgage secured by a 1-, 2- or 3-unit property that has one ADU.

The ADU must include a kitchen, bathroom and separate entrance and be independent of the primary dwelling; “the absence of cooking appliance(s) in a kitchen does not change the classification as an ADU.” Zoning must be legal, legal non-conforming, or no zoning.

An illegal ADU on a 1-unit subject may be eligible under 5601.2(c); rental income from an illegal ADU may not be used to qualify.

Manufactured-home ADUs are allowed under conditions (primary dwelling multi-wide; Bulletin 2025-15, settlement dates on or after 9 February 2026).

Appraisal. If the appraiser determines the subject has an ADU, the report must include a detailed description of the ADU, “any effect the ADU has on the market value or marketability of the subject property,” and specifics: general condition, room count including bedrooms and baths, finished square footage.

“At least one comparable sale with an ADU, when available, is required to demonstrate the property’s conformity and marketability to its Market Area.” If a recent comparable with an ADU is not available in the subject’s market area, “the appraiser can use an older sale with an ADU from the subject’s Market Area or a sale with an ADU from a competing Market Area as a comparable sale or as supporting market data.” The Guide FAQ on valuation (subcategory 3262, entries dated through 2 April 2026) asks, in terms, whether an appraiser may ignore the ADU or give it no value: no. “The appraiser’s analysis must be documented in the appraisal report and conclude whether an adjustment is supported for the ADU.” Supported is the word. Not assumed.

Rental income from an ADU on a subject 1-unit primary residence (Chapter 5306 / fact sheet). Purchase or “no cash-out” refinance only. Automated collateral evaluation (ACE), if offered, is not acceptable — a full appraisal is required.

Qualifying rental income documented with a lease “must not exceed 75% of the lease amount,” and “cannot exceed 30% of total income used to qualify.” The sales-comparison section must include at least one comparable sale with an ADU.

The ADU rental analysis must include at least three comparable rentals supporting the opinion of market rent, and at least one of those comparables must include a rented ADU. Form 1000 (Single-Family Comparable Rent Schedule) may be used for the ADU rental analysis.

For purchase transactions, at least one qualifying borrower must complete landlord education unless the borrower has a year of investment-property or ADU-rental management experience. Projected rent that will not start by the first mortgage payment — because the ADU is still being built — cannot be used.

HyreADU analysis: Freddie Mac has, since Bulletin 2022-11, been more explicit than Fannie Mae about the comparable-sale minimum (one ADU sale, when available) and about the rental-comparable minimum (three rents, one of them an ADU).

Those minima are a description of a thin market. A fact sheet that also says ADUs “may have the potential to increase long-term property and re-sale value” is a marketing sentence on a seller page, not a measurement, and we do not treat it as one.

The Guide is the rule; the fact sheet is the digest of the rule plus a slogan.

FHA 4000.1: legal status, and contributory value from the three approaches

HUD Handbook 4000.1 treats an ADU more tightly than the GSEs on legality, and more openly on method.

Definition (II.A.1.b.iv(B)(4) and II.D.3, as amended): “An Accessory Dwelling Unit (ADU) refers to a habitable living unit added to, created within, or detached from a primary one-unit Single Family dwelling, which together constitute a single interest in real estate. It is a separate additional living unit, including kitchen, sleeping, and bathroom facilities.” A one-unit property with a single ADU remains a one-unit property.

On a property that already has two or more units, a separate additional dwelling unit is counted as an additional unit — so a duplex plus an ADU is a three-unit property for FHA, not a two-unit with an accessory.

The mortgagee must obtain form HUD-92561 (Borrower’s Contract with Respect to Hotel and Transient Use) for a one-unit with an ADU.

Adomatis’s 2021 Exhibit 1 recorded the Agency split that still matters: VA and FHA require the ADU to be legal to be eligible; Fannie and Freddie, in the current B2-3-04 / 5601.2 texts, will finance some illegal ADUs with extra appraisal conditions and will not count illegal-ADU rent.

That is not a small difference. A unit that is financeable as a GSE accessory can be a deal-breaker on an FHA case if the zoning use is illegal.

Valuation (II.D.3, “Accessory Dwelling Unit”). The appraiser, as part of highest and best use, classifies the property as a single-family dwelling with an ADU or as a two-family dwelling.

ADU living area is not included in the primary dwelling’s gross living area. More than one ADU is an MPR/MPS deficiency the appraiser must flag to the mortgagee.

And the method sentence: “The Appraiser must measure the Contributory Value of the ADU by applying techniques based on one or more of the recognized three approaches to value: cost approach, income approach, and sales comparison approach.” FHA, unlike Fannie Mae B4-1.3-10, does not forbid using cost or income as a technique for the ADU line; it requires the appraiser to use whichever of the three is applicable.

It still requires the sales comparison approach for the property as a whole.

Mortgagee Letter 2023-17 (revisions to rental-income policies, property eligibility and appraisal protocols for ADUs) added the optional ADU market-rent analysis: Fannie Mae Form 1007 / Freddie Mac Form 1000 attached to the appraisal, with the supplemental statement that the form is completed to provide FHA an opinion of market rent of the subject’s legally rentable ADU.

Comparables used to develop that rent “must not include properties rented for hotel or transient purposes, or for periods less than 30 Days,” and “the Appraiser must include at least one comparable rental that is a Single Family dwelling with a rented ADU.” Same scarcity clause, written as a minimum.

California assessment: new construction, not a market premium

A California property-tax bill is not an appraisal, and the way an ADU hits that bill is a different statute from the GSE guides.

The statute is Proposition 13 (California Constitution Article XIII A) plus Revenue and Taxation Code Chapter 3, New Construction. It is California law. It is not how an Oregon or Texas assessor works.

Section 70 defines “newly constructed” and “new construction” as (1) any addition to real property, land or improvements, since the last lien date, and (2) any alteration of land or improvements that constitutes a major rehabilitation or that converts the property to a different use.

A major rehabilitation is work that converts the improvement to the substantial equivalent of a new one.

The Board of Equalization’s public new-construction page lists, as new construction, “additions to existing improvements,” “increasing the square footage of a residence,” “conversion of a garage, unfinished basement, or attic into a living area,” and “conversion of a single-family residence to a residence of two or more units.” A garage converted to an ADU is on that list. Painting, replacing a roof, and like-for-like fixture replacement are not.

Section 71 is the sentence that answers the fear of a full reassessment: “The assessor shall determine the new base year value for the portion of any taxable real property which has been newly constructed. The base year value of the remainder of the property assessed, which did not undergo new construction, shall not be changed.” New construction in progress on the lien date (1 January) is appraised at full value in its then-complete state each lien date until completion, when the newly constructed portion is reappraised and that value becomes its base-year value.

A supplemental assessment follows completion (Revenue and Taxation Code section 75 and following).

The Board of Equalization’s Assessors’ Handbook section AH 410, Assessment of Newly Constructed Property, and Property Tax Rule 463 (Newly Constructed Property) are the staff interpretation of those sections.

AH 531 (Residential Building Costs) is the cost manual many counties start from. None of those manuals is a sale-price study.

The enrolled increment is the assessor’s opinion of the full cash value of the new construction on the completion date, which in practice is often a cost approach — standardized regional costs for the quality class, not necessarily the owner’s invoice, and not necessarily what a buyer will pay for the unit on resale.

There is no ADU new-construction exclusion. SB 1164 (Newman, 2023–24) would have added Revenue and Taxation Code section 74.9 to exclude ADU construction completed between 1 January 2025 and 1 January 2030 from “new construction” for up to ten years, or until a change in ownership or a conversion to non-residential use.

The Board of Equalization’s bill analysis estimated an annual property-tax revenue loss of about $19 million.

The bill passed the Senate 29–6 on 22 May 2024 and died in the Assembly Revenue and Taxation Committee on 24 June 2024, hearing canceled at the author’s request. It is not law.

Builder sites that describe an ADU property-tax holiday under SB 1164 are describing a bill that failed.

The BOE’s published list of new-construction exclusions (sections 73 solar, 74.5 seismic, 74.3/74.6 disabled-access, 75.12 builder’s inventory, and the rest) does not include ADUs.

A resale story that assumes a tenant is wrong for a large fraction of owners

Value-add copy usually smuggles in a tenant. The GSE income rules above are written for a tenant. The income approach is written for a tenant. A large share of California ADUs do not have one.

Karen Chapple, Dori Ganetsos and Emmanuel Lopez, Implementing the Backyard Revolution: Perspectives of California’s ADU Owners (UC Berkeley Center for Community Innovation, 22 April 2021) — a survey of 752 California homeowners who had successfully built, identified from HCD Annual Progress Report addresses for 2018–2019 permits and certificates of occupancy — found that about half (51 percent) of California’s new ADUs serve as income-generating rental units, 16 percent provide no-cost housing to a relative of the homeowner, and 8 percent are short-term rentals.

David Garcia of the Terner Center reviewed that report; it is CCI’s survey, cited as CCI’s, with Terner in the acknowledgements.

Terner’s own cost and finance papers (Garcia 2017; Chapple, Garcia, Tucker and Valchuis 2020, Reaching California’s ADU Potential) are the complementary supply-side work. None of them is a sale-price premium.

That occupancy split is why the four numbers refuse to collapse. A unit occupied rent-free by a parent has a construction cost, an assessed increment, and a contributory market value that depends on whether the next buyer wants a second dwelling. It has no NOI.

Capitalising a market rent the owner will not collect, then calling the result “what the ADU is worth,” is a category error.

The ROI calculator already refuses a single number: it runs a tight, a base and a loose rent on the same cost, undiscounted and pre-tax, and a $0 rent is a valid input for family use.

The rental-income study and the rent digest keep HUD Fair Market Rent and ACS gross rent labeled as what they are.

What a homeowner can actually do with this

Ask the appraiser, not the internet, what the comparable set looks like on your block. The GSE guides have already told you the set may be old, or from the next town, or two illegal units and a listing. That is not a defective appraisal. That is the market.

Do not take a construction-cost invoice to a refinance and expect it back as appraised value.

Fannie Mae B4-1.3-10 will not accept cost as the sole indicator, and Brueckner and Thomaz’s just-sold subset — the closest thing California has to a market premium — is 9 percent in one city on a pre-boom panel, not a recovery of cost.

Budget the tax. Santa Clara, Los Angeles, San Francisco and San Diego will add an increment; the statute says they must; SB 1164 did not pass.

The increment is usually closer to cost than to a resale premium. Put that increment in the operating-cost field of the ROI calculator rather than pretending the bill does not change.

If the project only “works” as a 20–30 percent value-add, it does not work on the evidence this page could find.

If it works as housing for a person, or as a rent that still covers operating costs in the tight scenario, those are different, honest jobs.

The financing comparison is the page for how the money might be raised; this page will not originate a loan.

HyreADU analysis: the category has been answering a valuation question with a marketing number because the valuation evidence is thin and thin does not close a lead. Thin is still the evidence. We would rather publish the shorter honest page than a percentage we cannot source.