Quick answer: Yes, building or converting space into a legal ADU can increase a Maryland property’s assessed value, which can increase the property-tax bill. Maryland does not impose a special statewide ‘ADU tax.’ The tax effect comes from how the State Department of Assessments and Taxation (SDAT) values new improvements, additions, renovations and changes in the property.
Last verified: September 8, 2026. This guide is educational and is not tax, legal, appraisal or financial advice. Property assessments, credits and tax rates are property-specific.
Quick answer: what an ADU can change
| Question | Planning answer |
|---|---|
| Is there a special Maryland ADU property tax? | We found no separate statewide ADU property-tax surcharge in HB 1466. The relevant issue is the property’s assessed value after improvements. |
| Can an ADU increase assessed value? | Yes. SDAT says residential valuation considers property characteristics and new improvements or renovations. |
| Can the increase happen before the normal three-year reassessment? | Yes. A pending new-construction permit may trigger an out-of-cycle valuation when the permitted work adds more than $100,000 in assessed value or changes the use or character of the property. |
| Will every ADU create the same tax increase? | No. Assessment change, local tax rates, municipal taxes and credits differ by property and jurisdiction. |
| Does the Homestead Credit freeze the market value? | No. It limits how much of an assessment increase is taxable in a year when eligibility requirements are met; it does not prevent SDAT from determining a higher market value. |
How Maryland assesses improvements
Maryland real property is generally appraised by SDAT on a three-year cycle. SDAT’s assessment materials state that residential valuation considers characteristics such as the size, type and condition of the structure, construction quality, and new improvements or renovations.
For an ADU, that means the assessor is not simply looking at the amount you paid a contractor. The assessment process is concerned with the value and characteristics of the completed real-property improvement.
This is one reason our Maryland ADU Cost Calculator & Contractor Bid Comparison keeps project cost separate from future property value. Construction cost is a budgeting input; assessed value is determined through the state assessment process.
When an ADU can affect the assessment
SDAT’s Real Property Data Search guidance says a pending new-construction permit may result in an out-of-cycle revaluation if the permitted work produces more than $100,000 in added assessed value or a change in the property’s use or character. SDAT also says a reassessment permit can lead to an increase at the property’s next reassessment cycle, and can be converted to a new-construction assessment if inspection shows more than $100,000 in added value or a change in use or character.
The key distinction is important: $100,000 is not an ADU tax threshold and it is not the same as the contractor’s invoice. SDAT describes it in terms of added assessed value for out-of-cycle handling.
Assessment timing can also vary during the tax year. SDAT publishes new-construction cycles and notes additional quarterly cycles in several jurisdictions, including Baltimore City, Baltimore County, Howard County, Charles County, Montgomery County and Prince George’s County.
A simple property-tax example
Suppose an assessor ultimately adds $120,000 to the taxable assessment attributable to improvements, and the combined applicable real-property tax rate were hypothetically $1.20 per $100 of assessed value.
$120,000 ÷ 100 × $1.20 = $1,440 per year
That is only an illustration. It is not a Maryland-wide ADU estimate. County and municipal tax rates differ, the state publishes updated rate tables each year, the assessed-value change may be higher or lower than construction cost, and credits may affect the taxable amount.
How the Maryland Homestead Tax Credit fits in
For an eligible principal residence, Maryland’s Homestead Property Tax Credit limits the amount of an assessment increase that is taxable in a year. SDAT explains that every county and municipality must cap taxable assessment increases at 10% or less, with some local governments using lower caps.
The credit does not cap the market value that SDAT may assign to the property. It is a credit against part of an assessment increase when the homeowner and property continue to meet the program’s requirements.
One point deserves special attention for ADU owners: SDAT lists a substantial change in the use of the property among the conditions relevant to Homestead eligibility. The state’s ADU law does not tell homeowners how every rental or family-occupancy arrangement will be treated for Homestead purposes. Before relying on the credit in an ADU budget, ask SDAT how your specific ownership, occupancy and rental plan would be treated.
Does renting the ADU automatically determine the assessment?
Do not assume that rent collected from an ADU translates directly into a particular assessment increase. SDAT’s published residential assessment guidance emphasizes property characteristics, improvements and renovations, while its broader appraisal framework uses recognized valuation approaches as applicable. A property-specific assessor determines the value.
Also keep zoning and tax questions separate. HB 1466 allows ADUs to be offered for rent subject to the state framework and local rules, but it does not replace SDAT’s assessment rules. Short-term rentals are another separate issue: Maryland Department of Planning states that HB 1466 does not alter local governments’ authority to regulate short-term rentals.
Questions to answer before putting property tax in your ADU budget
- What is the property’s current assessed value? Check the SDAT Real Property database.
- When is the property’s normal reassessment year? Do not assume an ADU will wait until that date; qualifying new construction can be handled out of cycle.
- Could the permit result in more than $100,000 of added assessed value? That can matter to assessment timing.
- What county and municipal tax rates apply? Maryland publishes annual rate tables, but municipal property taxes can add another layer.
- Is the Homestead Tax Credit approved for the property? Check the property record and review eligibility before assuming the cap applies.
- Will the ADU change how the property is used? Ask SDAT about any Homestead or assessment consequence of the exact occupancy or rental arrangement.
- Are you budgeting the project separately from future value? Use the Maryland ADU Cost & Bid Tool for construction planning rather than assuming project cost equals assessment increase.
If you are still determining whether the project works on the property at all, start with the Maryland ADU Guide 2026, then use the Maryland County ADU Rules Tracker to verify the local ordinance.
Primary sources
- Maryland SDAT — Real Property Data Search definitions and new-construction assessment guidance
- Maryland SDAT — Homestead Property Tax Credit
- Maryland SDAT — Property tax rates and Homestead credit caps
- Maryland SDAT — 2025 Assessment Ratio Report
- Maryland Department of Planning — HB 1466 ADU FAQ
- Chapter 197, 2025 Laws of Maryland — HB 1466
Maryland ADU Guide is maintained by Gijo John as an independent research publication. See our Editorial & Research Policy for how regulatory and financial-planning pages are sourced and updated.