Thinking about adding an accessory dwelling unit to your Concord home? The rules shifted recently, and a smart design can be the difference between a smooth approval and months of revisions. You want a space that works for family or rental income and fits cleanly within zoning, building, and historic standards. In this guide, you’ll learn the by-right size limits, design moves that pass review, and the exact steps to get your ADU permitted in Concord. Let’s dive in.
What Concord allows today
Massachusetts now requires cities and towns to allow ADUs by right up to 900 square feet or 50 percent of the main home’s gross floor area, whichever is smaller. You must provide a separate entrance and meet building, fire, and health codes. See the statewide baseline in the state’s ADU overview and the recent law update.
- Key takeaway: Design to a 900 square foot by-right cap unless you confirm otherwise with the Town.
Helpful references:
- Review the statewide standards in the state’s ADU guidance at Accessory Dwelling Units and the ADU sections of Chapter 150 of the Acts of 2024.
- Concord’s 2025 Town Meeting approved Article 39 to update local ADU rules, including removal of owner occupancy and parking language aligned with state law. See the Article 39 page and the 2025 Town Meeting update.
Pick the right ADU type
Interior conversion
Basement, attic, or a portion of the main house often permits fastest and blends in visually. Watch ceiling heights, natural light, and emergency egress windows. A low-impact exterior helps when a property is near a historic area.
Attached addition
An addition can deliver privacy and accessibility while matching your home’s rooflines and materials. Plan the entry, massing, and window placement so the new volume feels secondary to the main house. Match finishes and proportions to reduce design friction in review.
Detached cottage or garage conversion
A backyard cottage or a garage-to-cottage build offers the most independence. Site it to meet setbacks and lot coverage, and use landscaping to screen views. Check Concord’s dimensional limits in the Zoning Bylaw.
Size and layout that pass review
Under state rules, the by-right limit is 900 square feet or half the main home’s gross floor area. See the definition of gross floor area in 760 CMR for how size is measured. Sleeping areas need compliant egress and the unit must have a direct entrance.
- Smart targets:
- Studio: 300 to 450 square feet with built-ins for storage.
- One bedroom: 450 to 700 square feet with a compact kitchen and full bath.
- Two bedrooms: possible within 900 square feet but plan carefully for egress and systems.
Review the size baseline in 760 CMR 71.02 and sleeping room egress in 780 CMR 1015.6.
Historic districts and neighborhood fit
Concord has several Historic Districts. Exterior changes in these areas typically require review by the Historic Districts Commission. Interior conversions that do not change the exterior may avoid HDC review. See process details at the Historic Districts Commission.
- Design to blend: echo the main home’s materials, simplify rooflines, and step down heights toward the street or neighbors.
Parking and site planning
State rules allow a town to require up to one on-site space for an ADU. No extra space is required if your lot is within one-half mile of qualifying transit. Concord’s MBTA Communities resources can help you check station areas and overlays. Start with the town’s MBTA Communities page.
- Tip: Where parking is required, consider permeable pavers and shared drive layouts to reduce paving and visual impact.
Septic, sewer, and utilities
If you are on municipal sewer, confirm connection details and fees. If you are on septic, a Title 5 review may be required and system capacity can limit bedrooms. Plan this early to avoid redesigns. Learn the basics at Title 5 septic systems.
- Early feasibility saves time and money. System capacity can dictate unit size and location.
Your permitting roadmap in Concord
Follow these steps to reduce surprises and keep your schedule on track.
- Zoning and overlays
- Confirm zoning district, setbacks, lot area, and floor area limits in the Concord Zoning Bylaw. Check if you are in a Historic District and whether MBTA station proximity could reduce parking.
- Preliminary feasibility
- Verify sewer or septic status. If septic, start Title 5 and soil testing. Decide whether your lot and goals point to an interior, attached, or detached ADU.
- Concept design
- Keep the by-right 900 square foot cap and separate entrance in mind. Sketch circulation, egress, and outdoor access. Plan screened parking if required.
- Documentation
- Prepare a scaled site plan showing the ADU footprint, access, parking, and utilities. Draft floor plans and elevations with gross floor area calculations. Coordinate HDC materials if applicable.
- Reviews and permits
- Confirm any needed HDC filing, provide Title 5 or sewer documentation, and submit your building permit package. Projects that meet by-right standards should proceed administratively.
Three smart floor plan ideas for Concord
420 square foot studio conversion
- Open living and sleeping, L-shaped kitchenette, full bath with a walk-in shower, stacked laundry in a closet. Uses egress window in sleeping area and minimal exterior change.
620 square foot one-bedroom addition
- Single-level layout with a no-step entry, 36-inch doors, U-shaped kitchen, and a 5-foot turning circle in the bath. Great for multigenerational living and aging in place.
850 square foot detached cottage
- Two-bed flex plan with vaulted living, compact galley kitchen, and a covered entry facing the backyard. Sited behind the primary home with low rooflines and native plant screening.
Mistakes to avoid
- Designing over 900 square feet without a clear special permit strategy.
- Ignoring septic capacity until late in the process.
- Placing entries or windows without considering neighbor privacy.
- Underestimating Historic District review for exterior changes.
- Forgetting the separate entrance and compliant egress paths.
Ready to shape an ADU that fits Concord’s rules and your goals? Thoughtful planning keeps you on budget and on schedule, and a right-sized design protects your approval path. If you want a steady, local advisor as you weigh value, layout, and resale, connect with Teri Adler for a calm, expert conversation.
FAQs
What size ADU can I build by right?
- By right in Massachusetts you can build up to 900 square feet or 50 percent of your main home’s gross floor area, whichever is smaller. See the definition in 760 CMR 71.02.
Do I need a separate entrance for the ADU?
- Yes. A separate exterior entrance or a code-compliant entry hall is required, and sleeping rooms must meet egress standards in 780 CMR 1015.6.
Is owner occupancy required in Concord?
- Concord’s Article 39 proposes removing owner occupancy, and state law limits such requirements. Check the current local bylaw text alongside the state’s ADU guidance.
Will I need extra parking?
- Towns may require up to one space, but no parking is required within one-half mile of qualifying transit. Confirm your location using the town’s MBTA Communities page.
What if my home is on septic?
- Start with a Title 5 review. If your system lacks capacity, an upgrade may be needed before occupancy. Learn more at Title 5 septic systems.
