Potomac + North Potomac
Larger lots make detached guest cottages and high-end custom ADUs realistic — but septic systems are common and change feasibility and cost.
Montgomery County, MD • detached ADUs • in-law suites • garage conversions
Montgomery County ADU builders — matched after you know the rules, not before.
MontgomeryADUBuilders.com is an independent planning and builder-match platform for homeowners researching detached backyard cottages, basement apartments, in-law suites, and garage conversions across Montgomery County. Understand feasibility first — then get matched with a licensed local builder.
Independent platform
MontgomeryADUBuilders.com is an independent homeowner education and contractor-matching platform. Featured contractors may compensate us for advertising and qualified introductions. Contractor participation does not constitute a government endorsement or a guarantee of workmanship. This site does not perform construction, design, or zoning determinations.
Project types
Match the structure to the lot, not just the dream.
Homeowners search in natural language first — backyard cottage, in-law suite, over-garage apartment, legal basement apartment — then discover the ADU rules afterward.
Detached backyard ADUs
Best for privacy, multigenerational living, or long-term rental. Must sit behind the principal home, meet greater setbacks, and typically require a fire-sprinkler system.
Detached ADU builders →Garage conversions
A strong fit for older homes and compact lots when a detached structure is unrealistic. Often faster when access, parking, and utilities cooperate.
Conversion rules →Basement or attached ADUs
Usually the most cost-efficient path when usable square footage already exists — but still needs code-compliant egress, fire separation, and a DHCA license.
Attached ADU rules →
Montgomery County rules
The county allows ADUs by-right — but the details decide whether a project pencils out.
Montgomery County has one of the more established ADU frameworks in Maryland. The practical constraints are usually footprint, setbacks, site access, parking, sprinklers, and how your lot interacts with detached accessory-structure rules.
◷ Last regulatory review: July 2026
Requirements may differ inside incorporated municipalities (Rockville, Gaithersburg, Takoma Park) and may change — Maryland's HB 1466 requires every county to allow ADUs on single-family lots by October 1, 2026. Confirm current rules with Montgomery County DHCA, DPS, the relevant municipality, and licensed professionals before purchasing plans or beginning work.
See full permit & rules breakdown with official county sources →
- One ADU per property. Montgomery County allows one accessory dwelling unit on a single-family lot.
- Owner-occupancy required. The property must be your primary residence — you may live in either the main house or the ADU. Airbnb / short-term rental is not permitted on a lot with an ADU.
- Detached size limit. The smallest of 10% of lot size, 50% of the primary-home footprint, or 1,200 sqft — and setbacks often become the real constraint (sometimes ~576 sqft in R-60 zones).
- Two departments. A DHCA Class 3 ADU license plus DPS building permits typically both apply. Takoma Park issues its own rental license; historic districts need a Historic Area Work Permit.
- Sprinklers & parking. Detached ADUs generally require a residential fire-sprinkler system; one added off-street space is typical unless within one mile of Metro/Purple Line/MARC.
Budget planning
Montgomery County ADU costs rise or fall on complexity, not just square footage.
Site work, utilities, fire sprinklers, retaining walls, drainage, tree protection, and design coordination are what move an ADU from “possible” to “expensive.”
$80k–$150kBasement ADU, when the space already exists and upgrades are moderate.
$120k–$200kGarage conversion, depending on structural and utility work.
$200k–$400k+Detached new-build ADU, most custom projects.
$180–$400Per square foot — regional planning range, by finish and complexity.
These are preliminary editorial planning ranges — not contractor estimates.
They are illustrative figures compiled from public Maryland/Montgomery County reporting as of July 2026, not quotes for your property, and they do not by themselves specify what is included. Design, survey and engineering, county and municipal approvals, site prep, utility connections, sprinklers, landscaping, financing, and contingency all sit on top. See the full cost breakdown and what drives each line →
Neighborhood coverage
Montgomery County is not one market.
The homeowner questions in Bethesda, Potomac, Silver Spring, Rockville, and Gaithersburg are not the same. Good local guidance reflects that difference instead of swapping city names into one page.
Bethesda + Chevy Chase
Compact, high-value lots; architectural fit matters; detached ADUs compete with premium additions and basements.
Bethesda ADU builders →Silver Spring + Rockville + Gaithersburg
Broader housing stock drives demand for garage conversions and basement apartments. Rockville and Gaithersburg administer some matters separately.
How builder matching works
What happens after you submit a review
- 1You share the basics.
Address, municipality, project type, intended use, budget range, and timeline — enough to screen feasibility without a site visit.
- 2We screen for fit.
We check the obvious constraints — municipality, ownership status, project realism — and grade the inquiry so builders see qualified projects.
- 3We match to a licensed builder.
Qualified projects are introduced to a vetted, licensed Montgomery County ADU contractor. You are told when a match is sponsored.
- 4You decide.
No obligation. You choose whether to move forward, get other opinions, or simply keep planning.
Why use this platform
The builder is only part of the decision. The site has to work first.
The best Montgomery County ADU projects start with feasibility discipline: can the lot physically accept the structure, can the numbers work, and can the team handle permitting cleanly. This platform is organized around those preconstruction questions.
What we do
Independent homeowner guidance, preliminary feasibility screening, and matching to licensed local builders.
What we don't do
We don't perform construction, design, zoning determinations, appraisals, or legal advice — and we say so plainly.
FAQ
Questions homeowners ask before they call.
Answers reviewed against Montgomery County DHCA and DPS materials in July 2026. Rules change — confirm current requirements before you build.
Can you build an ADU in Montgomery County by right?
Montgomery County generally allows ADUs as a by-right use on eligible single-family properties through DHCA and DPS, but the project still has to satisfy owner-occupancy, detached accessory-structure, setback, size, sprinkler, permitting, and licensing requirements.
How much does a detached ADU cost in Montgomery County?
Most detached custom ADUs fall in a preliminary $200,000 to $400,000+ planning range, driven by site work, utilities, fire sprinklers, design coordination, and finish level. These are editorial planning ranges, not contractor estimates — see the cost page for what's included.
Do I need a separate Montgomery County ADU license?
Typically yes. ADUs must be licensed through DHCA's Class 3 ADU process — whether or not they will be rented — and traditional building permits still come from DPS. Takoma Park issues its own rental license; historic districts require a Historic Area Work Permit first.
What is the maximum detached ADU size?
The smallest of 10% of the lot size, 50% of the primary home's footprint, or 1,200 square feet. In practice, setbacks often become the limiting factor — some R-60 lots are constrained to roughly 576 square feet.
Can I rent the ADU on Airbnb?
No. Short-term rentals such as Airbnb are not permitted on a property that includes an ADU, and the property must remain owner-occupied.
Start my ADU project review
Tell us about your property. We'll point you to the right next step.
Preliminary and educational — not a zoning determination, permit approval, appraisal, or legal advice.
Your project review is in.
Thanks — we've received your details and will review feasibility and match you with a licensed Montgomery County ADU builder. Watch for a confirmation at the email you provided. Planning something time-sensitive? Call (TRACKED-NUMBER).
Prefer to talk first? Call (TRACKED-NUMBER) or email hello@montgomeryadubuilders.com.