For years, the answer to “Can I build an ADU in Maryland?” depended almost entirely on which county you called home. That changed with new legislation in 2025, when the General Assembly passed House Bill 1466 and gave the state its first statewide framework for accessory dwelling units. Now that the October 1, 2026 deadline for local governments has passed, many homeowners across Central Maryland are asking what the Maryland ADU law actually allows on their property.
We’ve been remodeling homes for Maryland homeowners since 1958, and we’ve watched zoning rules shift many times over those decades. This change stands out. It gives families new ways to support aging parents, house adult children, or create flexible living space for the years ahead. It also carries more nuance than the headlines suggest.
Key Takeaways
- HB 1466 took effect October 1, 2025, and required every county and municipality to adopt a local law authorizing accessory dwelling units (ADUs) by October 1, 2026.
- An ADU can be an attached addition, a standalone structure, or in many jurisdictions a unit within the existing home, up to 75% of the size of the principal dwelling.
- HOA bylaws and deed covenants can no longer impose unreasonable limitations on building or renting an ADU, though limits on short term rentals are still allowed.
- Your county still controls the approval process, building permits, required setbacks, and many site requirements.
- Talking with an experienced ADU builder early helps clarify cost, permitting requirements, and feasibility before construction begins.
What the Maryland ADU Law Changed
Before this legislation, Maryland had no statewide standard for ADUs. Each jurisdiction set its own zoning rules, and the results varied widely from one county to the next. HB 1466 made it official state policy to promote and encourage ADUs on land where a single-family detached home is the primary dwelling, with the goal of expanding housing options and meeting the state’s housing needs.
The state definition is intentionally broad. An ADU is a secondary unit on the same lot as the primary home, and it can take several familiar forms:
- Attached ADU. An in-law suite built as an attached addition, often with its own entrance and a direct connection to the main home.
- Detached ADUs. Backyard cottages, granny flats, or a garage apartment set apart from the main residence.
- Conversions. A garage conversion or other accessory structures adapted into finished living space.
- Interior units. A basement apartment or other existing space within the house, which many jurisdictions also allow.
Any of these can be up to 75% of the size of the primary house. The Maryland Department of Planning has noted that counties cannot cap ADU size below that threshold, though a property owner is always free to build smaller.
The law also addresses private restrictions. Deeds, contracts, and HOA bylaws can no longer impose unreasonable limitations that prevent an owner from building an ADU or offering it for rent. HOAs may still restrict short term rentals, and properties listed in or eligible for the National Register of Historic Places are exempt from this provision.
How Local Zoning Rules Still Shape Your ADU
Many homeowners are surprised to discover that the state law did not give every property owner an automatic green light. It set minimum standards that each county’s current zoning code must meet, then left local governments substantial flexibility in writing their updated rules.
Here is how that balance generally works:
- Approval path. Counties may allow ADUs by right, or they may require a conditional use or special exception review.
- Safety and infrastructure. Local laws must require ADUs to meet building codes and adequate public facilities standards, including water, sewer, and septic capacity.
- Parking. A county cannot require an additional parking space for an ADU unless it first completes a parking study, and any off street parking space requirement must include a waiver process.
- Setbacks. Side and rear setbacks for an ADU cannot be stricter than those already applied to accessory structures such as detached garages.
- Alley access. A county may prohibit converting an accessory structure into an ADU when the only vehicle access is from an alley.
Counties can also be more permissive than the state requires, and they continue to set details such as height limits, lot coverage, and whether the owner must live on site. That variation is why two neighbors in different counties can receive very different answers about the same project.
ADUs in Baltimore County, Howard County, and Across Central Maryland
Several counties we serve, including Harford County, Baltimore County, Howard County, and Anne Arundel County, allowed some form of accessory apartments long before the state stepped in. For those jurisdictions, the new law prompted revisions rather than a fresh start. Howard County, for example, adopted Council Bill CB3-2026 this year, giving homeowners a broader path to build detached ADUs.
In our experience, the details that shape an ADU project in this region tend to be site-specific. A rural single family lot in northern Harford County or Cecil County served by a private well and septic system faces different questions than a suburban property on public utilities in Howard or Anne Arundel County. Septic capacity and utility hookups alone can determine whether a detached unit is practical, or whether an in-law suite within the existing footprint makes more sense.
County ordinances continue to evolve, so we always recommend confirming current requirements with your local planning and permitting office before finalizing design plans.
Planning Your ADU Project: Budget, Permits, and Design
An ADU is a complete home on a smaller scale. It needs a sleeping area, kitchen, and bathroom, along with the plumbing, electrical, and mechanical systems that support daily living. That scope is the main reason budgets vary so widely. Converting existing space inside the house usually costs less than building a standalone structure with new foundations, and new detached construction can reach into the hundreds of thousands of dollars depending on size, site work, and finishes.
A few items deserve attention as soon as you start planning:
- Building permits and fire sprinklers. Expect full building permits and inspections. Maryland also requires residential sprinkler systems in new single- and two-family dwellings, which includes newly constructed ADUs.
- Utility hookups. Connecting to water, sewer, or septic and confirming electrical service capacity can influence both design and cost.
- Who will live there. For family members such as aging parents, single-level living, wider doorways, and accessible bathrooms are far easier to plan from the start than to add later.
- Rental goals. Some Maryland residents view an ADU as a source of rental income. Review local licensing, parking, and owner-occupancy rules before design begins so the plan supports that goal.
- Design compatibility. A well-designed ADU should feel connected to the main home through rooflines, materials, and placement on the property.
Over the years, we’ve found that the most successful projects settle these questions before drawings are finalized. Permitting moves more predictably, selections stay on track, and the budget reflects the real scope of work. A thoughtfully built unit can also contribute to long-term property value by adding flexible living space that adapts as your household changes.
What to Look for in ADU Builders in Maryland
Building ADUs combines the complexity of a home addition with the requirements of a standalone residence. That makes the choice of contractor especially important. Look for ADU builders in Maryland who can speak clearly about county zoning, site constraints, and utility planning, along with finishes and floor plans.
A design-build approach helps here. When design and construction live under one roof, the team estimating your project is the same team drawing it, so budget conversations happen alongside design decisions. One thing we’ve consistently seen is that homeowners feel more confident when a single contractor coordinates permits, inspections, and trade schedules for the entire project.
Ask prospective contractors about comparable projects they have completed, how they handle permitting requirements, and how they communicate progress. Their answers will tell you a great deal about what working together will be like.
Taking the Next Step on Your ADU
The Maryland ADU law makes ADUs easier to approve in many communities, yet the right approach still depends on your lot, your county, and how you plan to use the space. A good first step is a feasibility conversation that looks at your property, local requirements, utilities, and what an ADU offers your household today and ten years from now.
For more than six decades, the Lynch family has helped Central Maryland homeowners plan home additions, in-law suites, and whole-home renovations with careful coordination and quality craftsmanship. We would be glad to help you explore whether an ADU fits your property and your goals.
Contact our design-build team to schedule a consultation, review your property, and map out a realistic plan.
Frequently Asked Questions
Do I need building permits for an ADU in Maryland?
Yes. Even under the new state law, an ADU in Maryland requires approval from your county or municipality, along with building permits and inspections. Some counties allow ADUs by right, while others require a conditional use or special exception review before construction can begin.
How large can an ADU be under the Maryland ADU law?
State law defines an ADU as up to 75% of the size of the principal dwelling. Counties cannot set a lower maximum, though you are always free to build smaller. Lot coverage, height limits, setbacks, and septic or sewer capacity can also influence how large your unit can practically be.
Can my HOA stop me from building an ADU?
HB 1466 prevents HOA bylaws and deed covenants from imposing unreasonable limitations on building or renting an ADU. HOAs can still restrict short term rentals, so it is wise to review your governing documents early in the planning process.
Can I rent out my ADU for income?
Generally, yes, subject to local rules. The state law protects the ability to rent an ADU against unreasonable private restrictions, but counties keep authority over short term rentals, and some apply owner-occupancy or rental licensing requirements.
Can I convert my garage into an ADU?
In many cases, yes. A garage conversion is one of the more common ways to add an ADU, and state law keeps counties from applying stricter side and rear setbacks than those used for other accessory structures. Counties may still prohibit conversions when the only vehicle access is from an alley.
How long does it take to build an ADU?
Timelines depend on the type of unit, the county review process, and the scope of site work. A conversion within the existing home often moves faster than a new detached structure. Design and permitting can take several months on their own, so we encourage homeowners to start planning early.