Last researched: September 19, 2026. An ADU can affect appraisal, marketability, financing and resale, but there is no credible rule that says an ADU adds a fixed percentage to a Maryland home’s value. The appraiser has to analyze the actual property, the ADU’s legal status, the local market, comparable sales and—when relevant—market rent.
This guide is separate from our Maryland ADU property-tax guide. Property-tax assessment and mortgage appraisal are different processes with different purposes.
What an appraiser is actually trying to answer
Is the ADU real and functional?
The appraiser needs to understand what exists: detached, attached or interior unit; size; condition; rooms; utilities; access; and how it functions relative to the main house.
Is the use legal or nonconforming?
Zoning status matters because legality can affect financing, marketability and the way comparable sales are selected.
Does the market recognize it?
The appraiser looks for sales, listings or other market evidence showing whether buyers in that market pay for similar ADU utility.
Can market rent be supported?
When a lender needs ADU rental income, the appraiser may need to estimate ADU market rent using true ADU rentals or appropriate substitutes.
Fannie Mae: ADUs must be analyzed, not guessed
Fannie Mae’s current Selling Guide says that when a property has an ADU, the appraisal report must describe the ADU and analyze any effect it has on the property’s value or marketability. The report must show that the improvements are acceptable for the market.
That is important because it rejects the shortcut homeowners often see online: “an ADU adds 20%” or “add construction cost to the home value.” Appraisal is market-based, not a simple cost-recovery formula.
Fannie Mae also says comparable sales should be selected based on similar physical and legal characteristics. That means the appraiser may consider site, room count, finished area, style, condition and other characteristics when choosing the best evidence.
What if there are very few comparable homes with ADUs?
This is one of the hardest appraisal problems for ADUs. In some Maryland markets, legal ADUs may still be uncommon enough that a perfect comparable does not exist.
Fannie Mae’s guidance allows flexibility. Its current ADU appraisal policy says an aged settled sale can qualify as a comparable, and an active listing or under-contract sale can be used as supplemental evidence to demonstrate marketability. The appraiser still has to explain the analysis.
Freddie Mac’s September 2026 ADU appraisal guide similarly emphasizes using the best available market evidence rather than pretending an identical sale must exist.
Can ADU rental income matter for financing?
Yes, under specific lender rules. Fannie Mae expanded its ADU policies in 2026. Its current guidance allows ADU rental income to be considered for qualifying in certain one-unit principal-residence purchase and limited cash-out refinance transactions.
Fannie Mae currently limits ADU rental income used for qualifying to no more than 30% of the borrower’s total qualifying income. When requested by the lender, the appraiser should develop an estimate of market rent for the ADU.
Fannie Mae says true ADU rental comparables should be used when available. When those are scarce, the appraiser may use appropriate substitute rentals, with careful consideration of the differences.
This is a lending-rule issue, not a promise that every borrower, lender or loan program will treat ADU rent the same way. Ask the lender which underwriting rules apply before relying on projected rent.
Legal ADU vs unpermitted or noncompliant ADU
| Status | Appraisal / resale implication | What the homeowner should keep |
|---|---|---|
| Permitted, finaled ADU | Cleaner documentation for appraiser, lender and buyer; market value still depends on evidence. | Permits, approved plans, final inspection/occupancy documents, recorded information where applicable. |
| Legal nonconforming / grandfathered use | Can still be marketable, but the appraiser/lender may need clear evidence of status. | Written zoning determination, prior approvals, permits, official records. |
| Unpermitted or zoning-noncompliant unit | Can complicate financing and appraisal; lender requirements become much more specific. | Do not rely on assumptions. Get written local guidance on legalization/removal/allowed use. |
Fannie Mae does not automatically make every zoning-noncompliant ADU ineligible. Its current guide allows some properties with illegal ADU use under specific conditions, including insurance confirmation and market evidence. It also requires the appraisal to identify the illegal use and support it with comparable sales showing the same noncompliant use.
That is not a homeowner workaround. It is a lender eligibility rule with conditions. If your unit is unpermitted or noncompliant, use our local rules pages and obtain jurisdiction-specific guidance before assuming it can be appraised or financed normally.
Does an ADU automatically increase resale value?
No. A legal ADU can increase buyer utility, but resale value depends on what buyers in that market actually pay for similar properties. Several factors can change the contribution:
- ADU size and condition;
- privacy and separate access;
- whether the layout feels independent;
- parking and site usability;
- legal/permitted status;
- rental potential;
- quality of construction and finishes;
- whether the ADU reduces useful yard, garage or parking space;
- local demand for multigenerational housing or rental flexibility;
- availability of comparable sales.
A poorly planned ADU can create tradeoffs. For example, converting the only garage may add dwelling utility but remove covered parking and storage. A detached ADU may add privacy but consume yard area. Appraisal is the net market reaction to the whole property, not simply the value of the new structure in isolation.
Cost does not equal value
If you spend $250,000 on an ADU, the appraised value does not automatically rise by $250,000. Construction cost and market value answer different questions.
Cost tells you what it took to create the improvement. Market value asks what a typical buyer would pay for the property in that market, considering the ADU and all other characteristics.
That distinction is why our ADU Cost Breakdown and this appraisal guide should be used together rather than treating one as a prediction of the other.
What to prepare before a refinance or sale
- approved permit set;
- final inspection or occupancy documentation;
- proof of zoning/legal status;
- accurate ADU square footage;
- bedroom/bathroom count;
- separate utility information if applicable;
- lease and rental history, if legally rented;
- photos showing condition and independent living features;
- receipts/warranties for major systems;
- any local rental license or registration documents.
Do not pressure an appraiser with your own “value add” calculation. Give clear factual documentation and let the appraiser determine how the market supports the adjustment.
Five questions to ask a lender before building for rental-income reasons
- Does this loan program allow ADU rental income to help me qualify?
- Does the ADU have to be permitted and finaled before the income can be used?
- What appraisal format and rental documentation will be required?
- Is there a limit on how much ADU income can count toward qualifying income?
- Would the rules differ for purchase, refinance or investment-property financing?
What changed in 2026
Fannie Mae expanded ADU eligibility effective March 31, 2026 for appraisals reported under UAD 3.6, including broader treatment of properties with multiple ADUs in certain circumstances. It also continued to allow qualifying rental income from one ADU under specified conditions.
Fannie Mae’s UAD 3.6 transition becomes mandatory on November 2, 2026. This matters because ADU reporting and rental information are being handled differently under the redesigned appraisal framework.
Freddie Mac published a new practical ADU appraisal guide on September 9, 2026, reflecting how mainstream ADU collateral analysis has become in mortgage underwriting.
Maryland ADU appraisal FAQs
How much value does an ADU add to a Maryland home?
There is no reliable statewide percentage. The appraiser analyzes the ADU’s characteristics, legal status, market acceptance and comparable sales to determine its effect on value and marketability.
Can an appraiser use rental income from an ADU?
Rental income can be relevant when the lender requests a market-rent analysis. Fannie Mae allows ADU rental income for qualifying in certain transactions, subject to specific limits and documentation rules.
Will an unpermitted ADU ruin an appraisal?
Not automatically, but it can complicate the appraisal and loan. Lender rules can require specific analysis, insurance confirmation and comparable evidence for zoning-noncompliant ADUs.
Do appraisers need another home with an identical ADU?
No. The appraiser uses the best available comparable evidence. Fannie Mae allows aged sales and supplemental listings in some ADU situations when better current evidence is unavailable.
Does building cost determine appraisal value?
No. Construction cost and market value are different. Spending a certain amount on an ADU does not guarantee an equal increase in appraised value.
Primary sources checked
- Fannie Mae — Improvements Section of the Appraisal Report
- Fannie Mae — Comparable Sales
- Fannie Mae — Appraiser Update, March 2026
- Fannie Mae — Rental Income from the Subject Property
- Freddie Mac — A Practical Guide to Appraising ADUs, September 9, 2026
Maryland ADU Guide is an independent research publication, not an appraiser, lender, mortgage broker, real-estate agent or financial adviser. Appraisal and underwriting decisions are property- and loan-specific. Confirm current requirements with the professionals handling your transaction.
What to do next
Start by finding the office that reviews your property. A mailing city or a county guide does not establish your parcel’s authority.
- Find Who Reviews My Property.
- Obtain your parcel identifier, survey, floor plans and utility records.
- Ask the reviewing office which current rules, supporting documents and approvals apply.
- Continue to your planning workspace.
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