The Complete Guide to ADUs on Martha’s Vineyard & Cape Cod (2025 Update)

What Is an ADU?

An Accessory Dwelling Unit (ADU) is a smaller, secondary home on the same lot as an existing single-family house. On Martha’s Vineyard and Cape Cod, ADUs have become incredibly popular as towns encourage housing options for families, caretakers, guests, and long-term residents.

ADUs can be:

• Detached (a separate small home)

• Attached (connected to the main house)

• Basement or garage conversions

• Bonus rooms or over-garage apartments

With new Massachusetts zoning support for ADUs, more homeowners than ever are considering them.

Why ADUs Are Growing Fast on the Vineyard & Cape Cod

Building an ADU locally offers several unique benefits:

1. Rental Income Potential

Coastal Massachusetts has high rental demand year-round—seasonal and long-term. Many homeowners build ADUs to create a steady income stream.

2. Guest or Family Housing

Perfect for:

• Aging parents

• Adult children

• Visiting family

• Caretakers or staff

3. Increased Property Value

A well-designed ADU can significantly increase the value of a Vineyard or Cape property, especially with square footage limitations across many towns.

4. Flexible Lifestyle

Home office, art studio, guest suite, or pool house — ADUs adapt as your life changes.

What Does an ADU Cost to Build in Massachusetts?

While costs vary by design and site conditions, here are realistic 2025 coastal Massachusetts numbers:

Typical Costs on Martha’s Vineyard & Cape Cod:

• Small ADUs (350–600 sq ft): $275,000–$350,000+

• Mid-sized ADUs (600–900 sq ft): $350,000–$500,000+

• Large ADUs (900–1,200 sq ft): $500,000–$700,000+

Why the range?Island logistics, shipping, limited subcontractor availability, septic adjustments, and zoning requirements all influence final pricing.

ADU Zoning Rules on MV + Cape Cod

Each town sets its own ADU regulations, but these points are consistent across most areas:

1. Size Limits

Most towns cap ADUs at 600–900 sq ft, though exceptions exist for multigenerational units.

2. Owner Occupancy

Some towns require the homeowner to live in either the ADU or main home.

3. Parking Requirements

Many towns require one additional parking space for the ADU.

4. Septic Considerations

Your septic system must be sized for the additional bedrooms, or you may need an upgrade or expansion.

5. Design Restrictions

Historic districts (like Edgartown, Vineyard Haven, Chatham, Wellfleet, etc.) may have visual and materials guidelines.

Before starting, it’s essential to confirm with your town’s building and zoning departments.

How Long Does It Take To Build an ADU?

On Martha’s Vineyard or Cape Cod, plan for the following rough timeline:

• Pre-Design & Permitting: 4–8 weeks

• Design & Engineering: 4–12 weeks

• Construction: 4–6 months (longer for complex sites)

The key to shortening this timeline is having permit-ready plans and a clearly defined scope before starting construction.

Should You Use an Architect or Pre-Designed Plan?

There are two main options:

Option 1: Custom Architect + Builder

Best when:

• Your lot is tight or unique

• You want a one-off high design

• You need a lot of creative freedom

Pros:

• Fully customized

• Fits site perfectly

Cons:

• Typically higher cost

• Longer design timeline

Option 2: Permit-Ready ADU Plans (Faster + Lower Cost)

Many coastal homeowners choose pre-designed ADU plans because they are:

• Faster to permit

• Lower cost than custom architecture

• Already code-compliant for Massachusetts

There are several plan providers available. One widely used resource in Massachusetts is MassADUPlans.com, which offers pre-engineered, permit-ready plans that simplify both the design and permitting phases. Homeowners often bring these plans to builders to speed up the process and reduce soft costs.(Note: We reference this company because it has become a common planning tool for Massachusetts homeowners—not because we own it.)

Steps to Building an ADU in Coastal Massachusetts

Here’s the exact process most Vineyard and Cape homeowners follow:

1. Evaluate Your Lot

Determine if your septic, zoning, and setbacks allow an ADU.

2. Choose a Design

Custom plans or permit-ready plans.

3. Engineering + Permitting

Structural engineering, Title V review, zoning approvals, building permit.

4. Set a Realistic Budget

Include:

• Site work

• Foundation

• Framing + carpentry

• Electrical, plumbing, HVAC

• Insulation

• Interior finishes

• Landscaping + hardscaping

5. Construction

4–6 months for a typical detached ADU.

6. Final Inspection + Occupancy

Confirm life safety compliance, septic approval, and final sign-off.

Tips for Building an ADU on Martha’s Vineyard or Cape Cod

• Work with a builder experienced in coastal construction — salt air, high wind loads, and sandy soils matter.

• Plan septic early — biggest hidden cost for ADU projects.

• Simplify the footprint — every corner, dormer, and roofline increases cost.

• Choose durable exterior materials like cedar, PVC trim, and hurricane-rated windows.

• Order long-lead items early to avoid island delays.

Final Thoughts: Is an ADU Worth It?

For most Vineyard and Cape homeowners, the answer is yes — whether for rental income, family use, or boosting property value.