Maryland ADU Rules Are Changing: What Montgomery County, Bethesda, Potomac & Rockville Homeowners Should Know

Could an ADU Help Your Home Work for You for Another 3–5 Years?

If you own a home in Montgomery County and are thinking about aging in place, downsizing, helping an aging parent, bringing family closer, or creating additional income, there is a major change in Maryland housing law you should know about.

Maryland is making accessory dwelling units—ADUs—easier to create.

An ADU might be a basement apartment, in-law suite, garage apartment, addition, or small backyard cottage.

Maryland law requires local jurisdictions to adopt ADU rules that comply with the state's new standards by October 1, 2026.

But I think homeowners should look beyond the new law.

The bigger question is:

Could an ADU make your current home work for you for another 3–5 years?

For some families, an ADU could mean Mom or Dad has an accessible place to live while remaining close to family.

It could give an adult child a private place to live.

It could provide space for a caregiver.

It might produce rental income that helps with the increasing costs of owning a home.

Or it could allow an older homeowner to move into a smaller, more manageable space while an adult child and their family occupy the main house.

For years, families have looked at aging in place as a choice between two options:

Stay in the house.

or

Sell the house.

Maryland's changing ADU rules create another question:

What if the property could work differently?

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What Is an ADU?

An accessory dwelling unit, or ADU, is a secondary independent dwelling located on the same property as a primary single-family detached home.

It generally contains its own facilities for sleeping, cooking, eating and sanitation.

An ADU can potentially be:

• inside the existing house;
• attached to the house;
• created from an addition;
• located above or inside a garage; or
• constructed as a separate backyard structure.

You may also hear people call these:

• In-law suites
• Mother-in-law suites
• Granny flats
• Basement apartments
• Garage apartments
• Backyard cottages
• Accessory apartments

But there is an important distinction.

A second kitchen and separate entrance do not automatically make a space a legal ADU.

Permits, zoning, licensing, building requirements and the property's legal status still matter.

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What Changed With Maryland's New ADU Law?

Maryland has established a statewide policy supporting ADUs and requires local jurisdictions to adopt compliant ADU laws by October 1, 2026.

The new state framework addresses many of the traditional obstacles that could make an ADU difficult or expensive to create.

Depending on the property and circumstances, these include:

• where ADUs may be permitted;
• lot-size restrictions;
• ADU size;
• setbacks;
• parking;
• density calculations;
• application processing;
• water and sewer connections; and
• certain HOA and private covenant restrictions.

The state is essentially saying:

If a property can reasonably accommodate an ADU, local rules should not create unnecessary barriers simply to prevent one from being built.

That doesn't mean every property qualifies.

Building codes, public health, utilities, septic systems, environmental restrictions and other legitimate property constraints can still matter.

The new law also changes how ADU applications are processed.

A complete zoning-use application generally must be approved or denied within 90 days.

And the zoning-use approval process is supposed to be objective and ministerial rather than discretionary, meaning the outcome should be based on whether the application meets the applicable standards rather than on a reviewer's judgment call.

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How Big Can a Maryland ADU Be?

Maryland's statewide definition generally limits an ADU to no more than 75% of the size of the primary dwelling, while requiring the ADU to remain subordinate to the main house.

That can produce surprisingly large numbers.

But don't start drawing a 3,750-square-foot backyard cottage.

The 75% standard is a ceiling within the state's definition, not a promise of what can be built on a particular property. It is only one part of the analysis.

There is also a size-related financial protection worth knowing about.

For an ADU under 750 square feet, Maryland generally prohibits a local development impact fee or building excise tax on the ADU.

For ADUs larger than 750 square feet, limitations on local development impact fees and building excise taxes still apply.

What can actually be built may depend on:

• The property
• Zoning
• Lot configuration
• Setbacks
• Building codes
• Existing structures
• Utilities
• Well and septic capacity
• Easements
• Environmental restrictions
• Stormwater requirements
• and other property-specific conditions.

The size of the house may tell us what is theoretically possible.

The property tells us what is actually practical.

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Why Are ADUs Important for Aging in Place?

Because aging in place is really about more than staying in the same house.

It's about whether the house continues to work.

I've seen versions of the same problem many times.

Mom wants to remain in her home, but her children don't want her living alone.

Dad is still independent, but the stairs are becoming difficult.

An adult child would happily move closer but doesn't want to move back into a childhood bedroom.

A caregiver is needed, but having someone living inside the main house feels intrusive.

The homeowner can afford the house today, but taxes, insurance, utilities, landscaping and maintenance continue to increase.

An ADU won't solve every problem.

But for some families, it creates another option.

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What If Mom or Dad Lived in the ADU?

This is where I think families should turn the traditional ADU idea upside down.

Most people picture:

Mom and Dad stay in the large house. The adult child moves into the ADU.

But what if the better arrangement is:

Mom or Dad moves into the smaller ADU while the adult child and family occupy the main house?

Now Mom or Dad may have:

• fewer stairs;
• less space to maintain;
• family just steps away;
• more privacy than moving into someone's spare bedroom;
• familiar surroundings;
• access to a neighborhood they already know; and
• a greater degree of independence.

The family may also avoid selling a longtime home immediately.

For the right property and family, that could extend the useful life of the property for several years.

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Could a Caregiver Live in an ADU?

Potentially, yes. An ADU can create private living space for a caregiver while keeping that person close to the homeowner.

That can be very different from having a caregiver occupy a bedroom inside the main house.

The homeowner retains privacy.

The caregiver has independent living space.

And help is nearby.

For some families, that arrangement may become increasingly valuable as needs change.

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Could an ADU Help Pay the Cost of Staying in the House?

Potentially. A legal ADU may create rental-income opportunities, depending on the property and applicable rules.

Rental income might help offset:

• Property taxes
• Homeowners insurance
• Utilities
• Landscaping
• Maintenance
• or Mortgage payments

But I would not build an ADU based solely on an online estimate of what it might rent for.

Becoming a landlord creates responsibilities.

Construction costs matter.

Insurance matters.

Taxes matter.

Maintenance matters.

And spending $200,000 creating an ADU does not automatically make your property worth $200,000 more.

The real question is:

Will the financial and lifestyle benefit justify what it costs to create the ADU?

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Will an ADU Increase My Property Value?

It may, but an ADU does not have a guaranteed dollar-for-dollar return.

A legal, well-designed ADU could make a property more appealing to buyers looking for:

• multigenerational living;
• an aging-parent suite;
• caregiver accommodations;
• adult-child housing;
• guest accommodations;
• flexible living space; or
• legal rental potential.

But there is another side to this.

A poorly designed conversion could eliminate garage space, reduce privacy, consume too much of the yard, or create a layout future buyers don't value.

Sometimes converting existing space produces the best return.

Sometimes building a detached ADU makes sense.

And sometimes the smartest financial decision is:

Don't build anything.

Before investing substantial money, I would want to understand how buyers are likely to value the finished property.

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Before You Build an ADU, Look Inside Your House

This may be one of the most important things on this page.

Before spending hundreds of thousands of dollars designing and constructing a backyard cottage:

Look at what you already own.

Do you have:

• A walk-out basement?
• A separate basement entrance?
• A full bathroom downstairs?
• An underused recreation room?
• An oversized attached garage?
• A detached garage?
• An existing second kitchen?
• A main-level area that could be reconfigured?

The least expensive ADU may not be something you build.

It may be space you already have.

That doesn't mean the space is automatically a legal ADU.

Its permit, building and licensing status still needs to be determined.

But I would investigate the existing house before building another one.

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COULD YOUR HOME WORK DIFFERENTLY?

Before assuming you need to move—or spend a substantial amount of money building an ADU—let's look at what you already have.

A basement, garage, main-level room or existing addition may create another option.

And sometimes a relatively simple modification to the main house may solve the problem without creating an ADU at all.

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What Are the ADU Rules in Montgomery County?

Montgomery County already permits ADUs, but its rules are being affected by Maryland's new statewide ADU requirements.

Montgomery County introduced Zoning Text Amendment 26-06 as part of bringing County regulations into alignment with the state law.

Areas addressed in the County's review include issues such as:

• where ADUs may be allowed;
• size standards;
• density calculations;
• setbacks; and
• off-street parking.

Because Maryland's compliance deadline is October 1, 2026, this is particularly important right now.

Do not rely on something you read about Montgomery County ADUs several years ago.

The rules are changing.

Verify the current requirements before spending money on plans or construction.

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Do Montgomery County ADUs Need a License?

Montgomery County currently has a Class 3 Accessory Dwelling Unit process, and licensing can apply even when an ADU isn't being rented to an unrelated tenant.

This surprises people.

They sometimes assume:

“My mother is living there. I'm not a landlord. Therefore this isn't an ADU issue.”

That isn't a safe assumption.

The physical characteristics and use of the space matter.

If you're creating an independent dwelling, determine exactly how Montgomery County classifies it before construction begins.

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Can My HOA Stop Me From Building an ADU?

Maryland's new law limits certain private restrictions that unreasonably interfere with otherwise lawful ADUs.

That can include certain restrictions contained in:

• declarations;
• covenants;
• HOA documents;
• bylaws;
• rules; and
• other instruments affecting real property.

A restriction that effectively prohibits an ADU or significantly increases the cost of creating one may raise an issue under the new law.

But don't interpret that as:

“My HOA can't tell me anything anymore.”

That's not what the law says.

Short-term rental restrictions, for example, are treated differently.

Other legal issues and exceptions can apply.

There may also be HOA assessment or voting implications associated with an ADU.

So the practical answer is:

Don't assume an old HOA restriction automatically ends the ADU conversation.

But don't ignore your HOA documents either.

If the HOA documents and Maryland law appear to conflict, that's a question for a qualified Maryland attorney.

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Will I Need a Separate Water and Sewer Connection?

Not necessarily. Maryland's new law limits when a utility can require a separate water or sewer connection for an ADU added to an existing property.

The law specifically includes WSSC within its utility provisions.

Where an existing connection is adequate, the state law can limit requirements for an entirely separate connection.

That's potentially important in Montgomery County.

But it does not mean adding an ADU comes with free utility work.

Plumbing, capacity, permits, construction and property-specific requirements can still cost money.

The important point is:

Don't assume a second dwelling automatically requires an entirely separate connection before investigating the property.

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What If My House Has Well and Septic?

This can completely change the conversation.

Some Montgomery County properties, particularly larger-lot properties, rely on private wells and septic systems.

An existing septic system may not have sufficient capacity for what you're proposing.

Drainfields matter.

Reserve areas matter.

Well locations matter.

Separation requirements matter.

And an ADU can potentially affect the wastewater demands placed on the property.

That creates one of the most important sentences in this article:

Zoning may say yes while the septic system says no.

Find out what serves the property before spending money on architectural plans.

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Can I Build an ADU in Bethesda, Maryland?

Potentially. Most Bethesda single-family properties fall under Montgomery County's ADU framework, but ADU potential varies substantially from one Bethesda property to another.

Bethesda is particularly interesting because some properties combine:

• older housing;
• finished basements;
• walk-out lower levels;
• garages;
• public water and sewer;
• proximity to Metro; and
• strong demand for flexible living arrangements.

That can make an existing-space conversion particularly interesting.

A Bethesda homeowner may not need to build a cottage in the backyard.

The opportunity could already be downstairs.

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What If My Bethesda House Already Has a Basement Kitchen?

Don't automatically call it an apartment.

And don't assume:

“The previous owner rented it for 25 years, so it must be legal.”

Maybe it is.

Maybe it isn't.

Before relying on that space for rental income—or advertising a property as having a legal apartment—verify:

• Permits
• ADU status
• Licensing
• Building requirements
• and Property records

Existing use and legal use are not necessarily the same thing.

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Does Being Near Bethesda Metro Matter?

It can. Transit proximity can affect the parking requirements applicable to an ADU.

That means two Bethesda houses only a few miles apart could present very different ADU opportunities.

A property closer to Metro might have a smaller lot but benefit from transit-related parking treatment.

A larger property farther from Metro may have more room for an ADU but different parking considerations.

So:

“I live in Bethesda. Can I build an ADU?”

isn't enough information.

We need to look at the property.

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Can I Build an ADU in Potomac, Maryland?

Potentially. Most Potomac properties are governed by Montgomery County rather than a separate municipal Potomac zoning code.

Potomac can present significant ADU opportunities because many properties have:

• larger homes;
• larger lots;
• space between houses;
• multigenerational households; and
• homeowners interested in remaining in their homes long term.

A large Potomac property may look perfect for a detached cottage.

But there is a catch.

A big yard does not automatically mean an easy ADU.

Before assuming a Potomac property can support one, determine:

• Public water and sewer or well and septic?
• Where is the septic reserve area?
• Are there easements?
• Are there environmental constraints?
• What is the zoning?
• Is there an HOA?
• Where could another structure actually fit?

A beautiful two-acre property can sometimes be more complicated than a much smaller Bethesda lot.

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Can I Build an ADU in Rockville, Maryland?

Potentially—but first determine whether the property is actually inside the incorporated City of Rockville.

A Rockville mailing address does not necessarily mean the property is governed by the City of Rockville's zoning rules.

Properties inside the City have their own ADU regulations and permitting process.

Rockville currently allows both attached and detached ADUs with qualifying single-unit detached homes.

Current City rules generally provide that:

• only one ADU is permitted per lot;
• the ADU may generally be up to 75% of the main dwelling's floor area, subject to additional rules;
• a basement ADU may occupy the entire basement;
• the owner must occupy either the main dwelling or ADU for at least six months of the year;
• the main house and ADU must remain under common ownership;
• an additional ADU parking space is not required; and
• existing and proposed ADUs must go through the City's permitting process.

Detached units have additional requirements involving the accessory building itself, including height and setbacks.

Rockville also has a specific rental accessory apartment licensing process when the secondary unit is offered for rent.

The first Rockville question isn't “Can I build an ADU?”

It's:

“Am I actually inside the City of Rockville?”

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Bethesda vs. Potomac vs. Rockville: Why Location Matters

The city or ZIP code is only the beginning.

ADU feasibility is property specific.

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Should I Build an ADU or Downsize?

This may be the most important question on the page.

If you're 60, 70 or 80 and thinking about what comes next, don't start with:

“How much does an ADU cost?”

Start with:

What problem am I trying to solve?

Do you want to remain in the house?

Do you need family closer?

Are stairs becoming difficult?

Do you need additional income?

Could you eventually need a caregiver?

Is the property becoming too expensive or time-consuming to maintain?

Once we understand the problem, we can compare the alternatives.

Option 1: Stay exactly as you are.

Maybe the house already works.

Option 2: Modify the existing house.

Perhaps creating a main-level bedroom and full bathroom solves the problem.

Option 3: Convert existing space into an ADU.

A basement, garage or existing addition may be the best solution.

Option 4: Build an ADU.

For the right property and family, this could create years of additional flexibility.

Option 5: Downsize.

Sometimes putting another $150,000, $200,000 or more into the existing property doesn't make financial or lifestyle sense.

There is no prize for remaining in a house that no longer works.

And there is no prize for moving before you need to.

The goal is to understand your choices while they are still your choices.

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Think 3–5 Years Ahead

Don't ask only:

“Can I live here today?”

Ask:

“Will this house still work for me 3–5 years from now?”

Think about:

• Stairs
• Bathrooms
• Bedroom location
• Mobility
• Driving
• Maintenance
• Property taxes
• Insurance
• Utilities
• Landscaping
• Being alone
• Family proximity
• Caregiving
• and Income

If the house isn't likely to work in its current configuration, an ADU may be one possible answer.

But it isn't the only answer.

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10 Questions to Answer Before Spending Money on an ADU

1. What problem are we trying to solve? Aging in place? Family housing? Caregiving? Rental income?
2. Can existing space solve it? Look at the basement, garage and main floor before building another structure.
3. Is the property eligible? Determine the jurisdiction, zoning and property-specific restrictions.
4. Is the property on public water/sewer or well/septic? Find this out early.
5. Is there an HOA? Review the documents in light of Maryland's new law.
6. What will the project actually cost? Get real estimates instead of relying on national averages.
7. What happens to taxes and insurance? Investigate both before construction.
8. What could the ADU realistically rent for? If rental income is part of the plan, use actual local market information.
9. What could the property be worth afterward? Construction cost and resale value are not the same thing.
10. What is the alternative? Compare the ADU with modifying the house, staying as-is, or selling and downsizing.

That final question matters.

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An ADU Is a Housing Strategy—Not Just a Construction Project

An architect may ask:

Can we design it?

A contractor may ask:

Can we build it?

A zoning professional may ask:

Can we get it approved?

All three questions matter.

But I want to answer something first:

Does doing this make sense for you and this property?

That's a different conversation.

Sometimes the answer may be:

Yes. Let's investigate the ADU.

Sometimes:

Modify the existing house instead.

And sometimes:

Don't invest another $200,000 into this property. Let's see what moving would give you.

The answer should come from your goals.

Not from someone trying to sell you a construction project.

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Could Your Montgomery County Home Support an ADU?

If you're considering an ADU in Bethesda, Potomac, Rockville or elsewhere in Montgomery County, I wouldn't start by hiring someone to draw plans.

I'd start with the property.

Let's look at:

• The house
• The lot
• The floor plan
• The basement
• The garage
• Utilities
• Potential property constraints
• Why you're considering an ADU
• What you're trying to accomplish over the next 3–5 years
• and Whether putting additional money into this property makes financial sense.

If the ADU idea looks promising, you can move to the appropriate zoning, permitting, design, construction, tax and legal professionals.

If it doesn't, you may have saved yourself a considerable amount of time and money.

Talk With Marc

If you're trying to determine whether you should modify your home, investigate an ADU, stay where you are, or downsize, let's look at the options before you make a major investment.

No pressure to sell your home.

Sometimes staying is the right answer.

Sometimes changing the house is the right answer.

Sometimes moving is.

The goal is to understand your options before circumstances make the decision for you.

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Frequently Asked Questions About Maryland ADUs

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Important Disclaimer

Marc Cormier is a real estate professional, not an attorney, architect, engineer, contractor, zoning official, accountant or tax advisor.

This page provides educational real-estate information intended to help homeowners identify questions and options worth investigating.

ADU laws and regulations are changing, and every property is different.

Before constructing, converting, renting or relying financially on an ADU, verify current requirements with the appropriate local government and qualified professionals.

For legal, estate-planning, HOA, title or other legal questions, consult a qualified Maryland attorney.