ADU Cost in Maryland in 2026

ADU costs in Maryland range from $80,000 for a basic garage conversion in Harford County to $298,000+ for a high-end detached unit in Montgomery County. The most common Montgomery County build — a 600 sq ft detached mid-range ADU — costs $150,000 to $210,000. Prince George's County runs 10–15% less. Baltimore City and the outer counties offer the most affordable construction at $85,000–$155,000 for a mid-range unit.

Maryland House Bill 1466, the Accessory Dwelling Units Act of 2025, became effective October 1, 2025. It requires every Maryland county and municipality with planning and zoning authority to adopt a local ordinance permitting ADUs on single-family lots by October 1, 2026. After that deadline, no Maryland jurisdiction can legally prohibit ADUs outright. Baltimore City, which currently lacks a clear citywide ADU ordinance, must comply. This is the most significant ADU legislation in Maryland history.

It varies by county. Prince George's County eliminated owner-occupancy requirements in its 2024 zoning reform. Montgomery County does not currently have a universal owner-occupancy requirement. Baltimore County has county-specific rules. HB 1466 does not mandate counties to eliminate owner-occupancy. Until all counties have adopted HB 1466-compliant ordinances (by October 2026), verify your specific county's current owner-occupancy rules before designing.

Maryland's Critical Area Act restricts development within 1,000 feet of the Chesapeake Bay and its tidal tributaries — affecting tens of thousands of residential properties in Anne Arundel, Calvert, Baltimore, and other coastal counties. ADUs on Critical Area lots require Critical Area Commission review in addition to county building permits. Impervious surface limits (typically 15–25% of lot) may restrict where an ADU can be placed or how large it can be. Check your parcel's Critical Area designation before designing.

A mid-range 600 sq ft ADU in Montgomery County costs $160,000 to $220,000 all-in and generates $1,700 to $2,200 per month in rental income. Cash-on-cash ROI runs 8–10%, with a payback period of 11–14 years. Prince George's County offers comparable ROI with lower build costs and no owner-occupancy requirement — making it the strongest net-investment ADU market in Maryland. DC proximity and federal employment provide rental stability unmatched in most markets.

Under HB 1466, Baltimore City must adopt an ADU-permitting ordinance by October 1, 2026. Baltimore currently lacks a clear citywide framework and most ADU applications require special approval. The city is expected to update its zoning code ahead of the state deadline. Once updated, Baltimore's large stock of historic rowhomes and rear-lot structures offers significant ADU conversion potential at relatively low cost ($85,000–$160,000 for a modest unit). Check with Baltimore City DHCD for the latest ordinance status.