Thinking about turning a basement into a rental, building an addition for family, or adding a small backyard cottage in Silver Spring? You are not alone. ADUs are a smart way to add flexible space or offset your mortgage, but figuring out rules, costs, and timelines can feel overwhelming. This guide breaks down how ADUs work in Montgomery County, what they typically cost, common use cases, and the first steps to check feasibility. Let’s dive in.
What is an ADU?
An accessory dwelling unit is a smaller, independent living space on the same lot as a primary home. It includes a place to sleep, a bathroom, and a kitchen or kitchenette.
ADUs can take several forms:
- Interior conversion: a finished basement, attic, or a portion of the main house.
- Attached ADU: a new addition that is physically connected to the home.
- Detached ADU: a separate small structure, often called a backyard cottage.
You will also hear terms like accessory building versus accessory dwelling unit, owner occupancy, parking requirements, setbacks, height limits, and lot coverage. These are zoning and building code concepts that shape what you can build and where it can go on your lot.
Where ADUs are allowed in Silver Spring
Silver Spring sits within Montgomery County. Whether you can build an ADU depends on your property’s zoning district, lot size and layout, whether you are on public sewer or septic, and any overlays such as historic districts or neighborhood conservation areas.
Start by confirming zoning and any overlays with the county’s planning staff. You can contact the Montgomery Planning Department for parcel-specific zoning guidance and to understand any master plan or design considerations. For the legal framework, you can review the county code through Municode’s Montgomery County library.
If your property is in a historic district or subject to a design review overlay, additional approvals may apply. Homeowner association covenants or deed restrictions can also be more restrictive than county rules, so review those documents early.
Permits and rules you will navigate
Most ADUs require permits and inspections. Expect to work with:
- Montgomery County Department of Permitting Services (DPS) for building permits, zoning compliance, and trade permits.
- Montgomery Planning for zoning verification or applicable design reviews.
- Health department review if you are on septic.
Common constraints and topics to confirm with county staff:
- Size limits: ADUs are often capped by square footage or by a percentage of the main home. Confirm the current limit for your zone.
- Owner occupancy and leasing: Rules can specify whether the owner must live on site. Check current requirements.
- Parking: Off-street parking may be required. Transit proximity can sometimes reduce or waive requirements.
- Short-term rentals: Many places restrict ADUs from being used for short stays. Verify the county’s policy before planning Airbnb-style use.
- Utilities: Detached units may need separate utility connections or metering. Interior units often share existing services.
- Fire and life safety: Egress windows, fire separation, smoke and carbon monoxide alarms, and sometimes sprinkler upgrades can apply.
- Taxes and assessment: Adding living area commonly increases assessed value and property taxes.
Timeline and fees: Zoning checks often take weeks. Permit issuance can take 4 to 12 or more weeks depending on scope and workload, with construction ranging from 2 to 9 months. Fees are typically based on project valuation and can range from several hundred to several thousand dollars. DPS staff can provide the most accurate, current timelines and fee estimates for your project.
ADU types and realistic costs
Here is what homeowners in the DC-area market typically consider, along with realistic cost ranges:
- Garage conversion: $25,000 to $100,000. Often the lowest-cost detached option but may require foundation, insulation, and structural upgrades. You also lose covered parking.
- Basement or attic conversion: $50,000 to $150,000 for a full unit with kitchen and bath. Egress, ceiling height, and natural light drive cost.
- Simple interior conversion (small studio or JADU-style): $20,000 to $70,000 when scope is limited.
- Attached ADU or addition: $100,000 to $300,000+. Offers design control and integrated systems but adds structural and exterior work.
- Detached cottage (modular or stick-built): $120,000 to $400,000+. Site work, utilities, and foundations can move costs toward the high end.
Cost drivers include scope and finishes, site conditions and grading, tree protection and stormwater requirements, utility runs, historic or design review, accessibility features, and local labor costs. Getting a preliminary estimate from local architects and contractors is essential once you define your target size and layout.
If you are still shaping your plan, national guides like AARP’s ADU resources and HUD case studies can help you think through design, access, and cost tradeoffs.
How Silver Spring homeowners use ADUs
- Multigenerational living: Space for aging parents or adult children while maintaining privacy and independence.
- Long-term rental income: A separate unit can produce steady rent that helps offset your mortgage and monthly costs.
- Home office or studio: Flexible space that can convert to a rental later if needs change.
- Caregiver housing or accessibility planning: A ground-level or adaptable layout can support changing mobility needs.
- Short-term rental: Often restricted. Confirm whether ADUs are eligible for short-term stays under current county rules.
A simple rent math example
Run the numbers conservatively before you commit.
Example assumptions:
- Monthly rent: $1,700
- Vacancy and credit loss: 8 percent
- Owner-paid utilities, maintenance, insurance, taxes allocated to ADU: $250 per month
Math:
- Annual gross rent: $1,700 x 12 = $20,400
- Less vacancy: 8 percent of $20,400 = $1,632
- Effective gross income: $18,768
- Less operating costs: $250 x 12 = $3,000
- Estimated net income before financing: $15,768 per year
Compare this to your total project cost to estimate a simple yield. For example, a $150,000 project with $15,768 in net income would show about a 10.5 percent gross yield before financing and taxes. Your actual results will vary, so build in a contingency and get multiple contractor bids.
Feasibility checklist for Silver Spring
Use this step-by-step checklist to scope your project:
Confirm zoning and overlays
- Contact Montgomery Planning for your property’s zone and any overlays.
Verify sewer versus septic
- If on septic, ask DPS or the health department about capacity or upgrades.
Check lot constraints
- Lot size, setbacks, lot coverage, trees, and slope.
Choose your ADU type
- Interior conversion, garage conversion, attached addition, or detached cottage. Note tradeoffs like parking loss or tree removal.
Check parking and transit exemptions
- Confirm off-street parking requirements and any transit-based reductions.
Historic district or design review
- If applicable, start early to align design and avoid delays.
Plan utilities and mechanicals
- Assess water, sewer, electric, and gas capacity. Ask about separate meters for detached units.
Review HOA or deed restrictions
- Check covenants for rules on rentals or accessory structures.
Build a budget and financing plan
- Consider cash, HELOC, cash-out refinance, renovation or construction loans. Lender requirements for ADUs can vary.
Map your permitting path
- Ask DPS for the permit checklist and submittal requirements.
Select experienced pros
- Choose a contractor and architect with Montgomery County ADU experience and references.
Set timeline and contingencies
- Account for county review, inspections, weather, and a 10 to 20 percent budget contingency.
Prepare for leasing and compliance
- If you plan to rent, learn local landlord-tenant rules, update insurance, and put systems in place for screening and maintenance.
Local resources and next steps
- Start permit and inspection planning with the Montgomery County Department of Permitting Services.
- Confirm zoning, overlays, and planning guidance with Montgomery Planning.
- Review the law directly via Montgomery County Code on Municode.
- Explore design and planning primers from AARP Livable Communities and HUD resources.
If you want a second set of eyes on ADU potential before you renovate or buy, reach out. We help you evaluate lot constraints, resale implications, and the best path to value. Contact The Foley Group to talk through your goals and get connected with local architects and contractors.
FAQs
Can I build an ADU on my Silver Spring lot?
- It depends on your zoning, lot size, sewer or septic status, and any overlays or HOA rules. Confirm eligibility with Montgomery Planning and check permitting with DPS.
How much does an ADU cost in Montgomery County?
- Typical ranges: $20,000 to $70,000 for simple interior conversions, $50,000 to $150,000 for full interior units, $100,000 to $300,000+ for attached additions, and $120,000 to $400,000+ for detached cottages.
Are short-term rentals allowed in ADUs in Montgomery County?
- Short-term rentals are often restricted for ADUs. Verify current county policy and any HOA rules before planning for short stays.
Do I need to live on the property if I add an ADU?
- Owner-occupancy rules vary by jurisdiction and can change. Confirm the current requirement with county staff for your property and ADU type.
How long do permits and construction take for an ADU?
- Zoning checks often take weeks. Permitting can take 4 to 12 or more weeks depending on scope, with construction typically 2 to 9 months based on complexity and contractor availability.
