Frequently Asked Questions
Maryland ADU answers.
How much does an ADU cost in Maryland?
ADU costs in Maryland range from $70,000 for a garage or basement conversion to $380,000 for a large detached new build in Montgomery County. The most common project — a 600–700 sq ft 1-bedroom detached ADU — falls in the mid-range for your region. See the regional breakdown above for market-specific data.
Does Maryland have a statewide ADU law?
Yes. Governor Wes Moore signed SB 891/HB 1466 (Chapter 196/197 of 2025) on April 22, 2025, effective October 1, 2025. It requires every county and municipality with zoning authority to adopt a compliant local ADU ordinance by October 1, 2026, that authorizes ADUs on land with a single-family detached home, excludes them from density and residential growth-limit calculations, and caps side/rear setbacks at existing accessory-structure standards. Beyond those three requirements, localities keep discretion over height, design, fees, and owner-occupancy. Local zoning, building, utility, site, and private-covenant requirements may still apply; verify the current rules for the property before design or construction.
What happens if a Maryland county doesn't meet the October 1, 2026 ADU deadline?
Nothing automatic. Chapter 196/197 of 2025 sets the October 1, 2026 compliance deadline for local governments but includes no penalty, no enforcement mechanism, and no default rule that legalizes ADUs if a county or municipality misses it. Earlier drafts of the bill included a 90-day ministerial permit-approval requirement, impact-fee exemptions for units under 750 sq ft, and utility-connection protections — all three were removed before final passage. Until a locality adopts its own compliant ordinance, its existing local rules remain the operative checkpoint for ADU permitting.
What makes Montgomery County ADU permitting unique?
Montgomery County has its own comprehensive ADU ordinance with specific design standards, setback requirements, and size limits beyond the state minimums, plus a dedicated ADU review process. Pre-application meetings are strongly recommended — planning staff can flag issues before you invest in full architectural drawings.
Can I build an ADU in Baltimore City?
Yes. Baltimore City allows ADUs in most residential zones under its existing zoning code, and the city has an established tradition of basement apartments in its rowhouse neighborhoods. Budget 4–6 months for permit review — Baltimore's process can run slower than the suburban counties.
Does Maryland require owner-occupancy for ADUs?
Owner-occupancy treatment remains a local checkpoint during implementation of HB 1466 / Chapter 197 (2025). Confirm the current ordinance and application requirements with the city or county before relying on an owner-occupancy assumption.
What is the Chesapeake Bay Critical Area?
Properties within 1,000 feet of tidal waters face additional development restrictions, including limited impervious surface coverage. If your property is in the Critical Area, ADU construction may be restricted or require mitigation — check your classification with your county planning department before proceeding.
What is the ROI on an ADU in Maryland?
The adopted Maryland one-bedroom planning range is $900–$1,875 per month, derived from the HUD FY2026 revised FMR market basket under ROI-CM-1.4. Payback is calculated from the homeowner's budget, ADU type, size, rent range, and a 30% operating-expense reserve. Property value uses a 15% national planning input with a 10%–20% uncertainty range; it is not a valuation.
