Town Permit Guide

Home › Massachusetts › Dennis › Accessory Dwelling Units (ADUs)

Can I build an ADU in Dennis, Massachusetts?

Dennis, Barnstable County · Bylaws checked 2026-10-15 · Reviewed · 8 cited sections

Allowed with a permit

Yes. Dennis allows one "Protected Use" ADU by right (building permit only) on a lot with an existing home in every zoning district except Industrial, as long as it is no larger than 900 sq ft or half the main home's floor area, whichever is smaller. On a lot with a protected ADU, neither the ADU nor the main house may be rented short-term: leases must be longer than 31 days. ADUs over 900 sq ft need a Planning Board special permit, must be inside or attached to the house, and carry owner-occupancy and 12-month lease rules.

ADUs allowed by right1 Protected Use ADU per lot, in all zoning districts except Industrial [8]
Max size (by right)900 sq ft or 1/2 the main home's gross floor area, whichever is smaller [1]
Short-term rental of a protected ADUProhibited; leases must be over 31 days (both the ADU and the main house) [2]
Larger ADU (over 900 sq ft)Special permit (Planning Board); must be inside or attached to the house [3]
Special-permit ADU minimum lot size15,000 sq ft [4]
Special-permit ADU owner occupancyOwner lives in one unit year-round; other unit leased 12+ consecutive months [5]
Parking (bylaw schedule)2 spaces for the main home + 1 per ADU bedroom (state law caps a protected ADU at 1 extra space) [6]

Protected (by-right) ADUs

Section 4.11.A follows the 2024 state ADU law: one Protected Use ADU is allowed per lot [1]. The use table marks it "Y" (allowed) in every district column except Industrial [8]. The text of §4.11.A lists the districts too, but that list has apparent typos ("LL", "DPVC Area BA"), so the use table is the clearer guide. The ADU must be a self-contained unit with its own sleeping, cooking and sanitary facilities and a separate entrance that meets building-code egress rules, and its gross floor area may not exceed half the main home's or 900 square feet, whichever is smaller [1]. The bylaw doesn't say it must be attached, and state law allows detached ADUs. It can only be added to a lot that already has a principal dwelling, it can never be sold off separately, and it is subject to the bylaw's lot coverage, setback and dimensional rules plus Board of Health and Conservation Commission requirements. The Building Inspector reviews architectural plans at building-permit time to confirm the design is consistent with the main house [7].

No short-term rentals

A protected ADU may not be used as a short-term rental, and both the ADU and the main house must be leased for more than 31 days at a time [2]. State law lets towns restrict short-term rental of ADUs. It doesn't clearly address a limit on the main house, so confirm that part with the town. EOHLC has proposed changes to the state ADU regulations that could affect minimum-lease rules, but they are not final.

ADUs by special permit

An ADU over 900 sq ft, or one that otherwise doesn't qualify as protected, needs a special permit from the Planning Board [3]. These must be inside or attached to the existing home, the lot must be at least 15,000 sq ft [4], only one ADU is allowed on the lot, parking must be on site, and the unit can't be in an AE or V flood zone. The owner must live in one unit year-round, and the other unit must be leased for at least 12 consecutive months, with no seasonal, weekly or vacation rentals [5]. The leased unit also needs a Board of Health rental permit, and the owner files an affidavit and lease with the Building Commissioner every January 31. Special-permit ADUs are capped at 9 per year through 2027. Section 4.11.A(g) mentions a second ADU by special permit, but §4.11.4.2 says a special-permit ADU can't be on a lot with another ADU, so a second ADU doesn't appear possible.

Parking

The parking schedule calls for 2 spaces for the main home plus 1 per ADU bedroom [6], and §4.11.A makes protected ADUs subject to the parking rules [1]. State law says a protected ADU can't be required to have more than 1 extra space (none near transit), so state law may override this schedule for by-right ADUs with more than one bedroom.

See how Dennis compares with 8+ other Massachusetts towns on accessory dwelling units (adus) →

Next steps

  1. Confirm your zoning district and floor areas: the ADU must be no more than 900 sq ft and no more than half the main home's gross floor area to qualify by right.
  2. Check septic capacity (Title 5) and wetlands with the Dennis Board of Health and Conservation Commission before designing.
  3. Apply to the Dennis Building Department (508-760-6157) for a building permit, with architectural or design plans showing the ADU matches the main house.
  4. If the ADU is larger, a second unit, or in a non-single-family district, apply to the Planning Board for a special permit, with deed, owner-occupancy affidavit, and lease documents.

Common questions

Can I build an ADU in Dennis, MA?
Yes. Dennis allows one protected ADU by right, with just a building permit, on a lot with an existing home in every zoning district except Industrial. The bylaw doesn't say it must be attached, and state law allows detached ADUs.
How big can an ADU be in Dennis, Massachusetts?
A by-right ADU can be no larger than 900 sq ft or half the main home's gross floor area, whichever is smaller. A larger ADU needs a Planning Board special permit, must be inside or attached to the house, and requires a lot of at least 15,000 sq ft.
Can I Airbnb my ADU in Dennis, MA?
No. On a lot with a protected ADU, neither the ADU nor the main house may be rented short-term; leases must be longer than 31 days. State law doesn't clearly address the limit on the main house, so confirm that part with the town.
Does the owner have to live on site for an ADU in Dennis, Massachusetts?
Only for special-permit ADUs. For those, the owner must live in one unit year-round and the other unit must be leased for 12 or more consecutive months. By-right protected ADUs don't carry that rule.
How much parking does an ADU need in Dennis, MA?
The bylaw schedule calls for 2 spaces for the main home plus 1 per ADU bedroom, but state law caps a protected ADU at 1 extra space.

Massachusetts' statewide ADU law

Since February 2, 2025, the Affordable Homes Act (St. 2024, c. 150, §8) has protected one accessory dwelling unit (ADU) per lot in any zoning district that allows single-family homes (M.G.L. c. 40A, §3). Your town cannot ban it, require a special permit or other discretionary approval, or "unreasonably restrict" building or renting it.

What counts as a protected ADU:

  • A self-contained unit with its own sleeping, cooking and bathroom facilities, on the same lot as the main home.
  • No larger than 900 sq ft or half the main home's floor area, whichever is smaller.
  • It can be attached or detached, and it needs a separate entrance that meets the building code.

What towns cannot require:

  • That the owner live in the main home or the ADU.
  • More than 1 extra parking space, or any extra space if the lot is within 0.5 miles of a subway, commuter rail, ferry or bus station.
  • A minimum lot size, caps on the number of ADUs, or limits on who may live there (age, income, relationship).

What towns can still do: use reasonable site plan review, setback, height and bulk rules, require Title 5 septic compliance, and restrict or ban short-term rentals of ADUs. A second ADU on the same lot needs a special permit. State regulations at 760 CMR 71.00 (in effect since January 31, 2025) explain which rules count as unreasonable.

State sources: M.G.L. c. 40A, §3 (protected ADU paragraph) · M.G.L. c. 40A, §1A (ADU definition) · EOHLC, Accessory Dwelling Units (law, FAQ, 2026 draft regulations) · 760 CMR 71.00, Protected Use Accessory Dwelling Units

What the bylaw says

Exact text from Dennis's published bylaws. Section numbers are as printed in the source.

  1. § 4.11.A, Dennis Zoning By-Law (amended through and including ATM May 6, 2025), p. 77
    Protected Use ADU is defined as a self-contained housing unit, inclusive of sleeping, cooking and sanitary facilities on the same Lot as a Principal Dwelling, subject to otherwise applicable dimensional and parking requirements, that: (i) maintains a separate entrance, either directly from the outside or through an entry hall or corridor shared with the Principal Dwelling sufficient to meet the requirements of the state building code for safe egress; (ii) is not larger in Gross Floor Area than 1/2 the Gross Floor Area of the Principal Dwelling or 900 square feet, whichever is smaller and is proposed to be located on a Lot in a Single-Family Residential Zoning District and no other Accessory Dwelling Unit is located on said Lot.
  2. § 4.11.A(a), Dennis Zoning By-Law (amended through and including ATM May 6, 2025), p. 78
    Any Protected Use ADU created shall be prohibited from serving as a Short-Term Rental defined in General Laws Chapter 64G, Section 1. The minimum leasing term for the Accessory Dwelling Unit or the Principal Dwelling Unit shall be for a term greater than thirty-one (31) days.
  3. § 4.11.A(g), Dennis Zoning By-Law (amended through and including ATM May 6, 2025), p. 78
    For ADU’s not allowed by right, such as more than one (1) ADU 900 square foot or less in a single-family residential zoning district, or an ADU larger than 900 square feet in a single-family residential zoning district, or an ADU in an any district other than a single- family residential zoning district, there shall be a Special Permit for the use of land or structures for an accessory dwelling unit per § 4.11 of the Dennis Zoning Bylaws.
  4. § 4.11.4.1, Dennis Zoning By-Law (amended through and including ATM May 6, 2025), p. 79
    4.11.4.1 Lot must be a minimum of 15,000 sf.
  5. § 4.11.9, Dennis Zoning By-Law (amended through and including ATM May 6, 2025), p. 80
    The principal dwelling or the ADU will be used as the principal residence of the owner and the remaining dwelling will be leased for a minimum of twelve (12) consecutive months, with no subletting or assigning to occur and is prohibited from any use as rental units on a monthly, weekly or daily basis including, but not limited to, seasonal rental and rental through vacation rental services and websites.
  6. § 3.1.3 Off Street Parking Schedule, Dennis Zoning By-Law (amended through and including ATM May 6, 2025), p. 35
    Accessory Dwelling Unit 2 for primary residence + 1 per bedroom in accessory dwelling unit
  7. § 4.11.A(f), Dennis Zoning By-Law (amended through and including ATM May 6, 2025), p. 78
    A Protected Use ADU shall be designed so that the appearance of the ADU will be consistent with the character and design of the principal dwelling as much as feasibly possible, as determined by the Building Inspector. To ascertain this, architectural or design plans must be submitted at the time of application for a Building Permit.
  8. § 2.2.2 Use Regulation Schedule, row 2.L, Dennis Zoning By-Law (amended through and including ATM May 6, 2025), p. 12
    L. Protected Accessory Y Y Y Y Y Y Y N Y Y
This page summarizes Dennis's posted bylaws as of 2026-10-15. Bylaws are amended, and individual lots can have overlays, subdivision covenants, or HOA rules that are stricter. Confirm with Building Department (Building Commissioner / zoning enforcement) (508-760-6157) before you build. Spot an error?