How Much Does It Cost to Build an ADU in Massachusetts?

A 2025 Cost Breakdown for Homeowners Exploring Accessory Dwelling Units

Accessory Dwelling Units — better known as ADUs — are quickly becoming one of the most practical and in-demand housing options in Massachusetts.

With the new statewide ADU law in effect, homeowners can now add in-law suites, rental cottages, or backyard guest houses with far fewer restrictions than ever before.

But before you start planning, there’s one key question:How much does it actually cost to build an ADU in Massachusetts?

As a builder working throughout Cape Cod, Martha’s Vineyard, and the South Shore, we’ve seen a wide range of projects. The numbers below reflect what you can realistically expect in 2025 — and why early planning makes all the difference.

Average ADU Construction Costs in Massachusetts

While every property is different, most homeowners can expect the following price ranges:

• Small Detached ADU (400–600 sq. ft.): $180,000–$250,000

• Medium Detached ADU (600–800 sq. ft.): $225,000–$325,000

• Large Detached ADU (800–900 sq. ft.): $300,000–$400,000+

• Attached or Basement ADU: $150,000–$275,000 (depending on existing structure)

These figures generally include the full build — foundation, framing, utilities, and interior finishes — but exclude items like landscaping, septic upgrades, or major site prep.

What Affects the Cost of an ADU

1. Size and Layout

Smaller footprints with simple, rectangular layouts are the most cost-effective.More complex rooflines, vaulted ceilings, or high-end finishes increase both material and labor costs.

2. Detached vs. Attached

Detached ADUs are standalone structures with their own foundation and utility runs, while attached or basement ADUs share existing systems.Detached units cost more but offer more privacy and flexibility.

3. Site Conditions

Soil type, topography, and access matter.In coastal areas like Cape Cod or Martha’s Vineyard, factors such as staging, ferry logistics, and equipment access can add 10–15% to the total budget.

4. Septic and Utility Requirements

If your property is on septic, you’ll likely need an engineer to confirm capacity.A system upgrade or new tank can add $20,000–$50,000 depending on lot conditions and soil.

5. Level of Finish

Like any home, finishes make a huge difference in cost. Luxury tile, custom cabinetry, and built-ins can quickly move an ADU from $250K to $400K+.

Design & Permitting Costs

With the new ADU law, Massachusetts now allows ADUs by-right in most single-family zoning districts — meaning no special permit in most cases.

That said, homeowners still need complete architectural drawings, engineering, and building permits to start construction.

For those not ready to invest in a custom design, there are excellent plan resources available — one we often refer clients to is MassADUPlans.com.

They offer permit-ready ADU plan sets tailored specifically for Massachusetts building codes and zoning standards.Homeowners can select a pre-designed ADU, purchase the plan set, and customize it to fit their lot — all for a fraction of the cost of starting from scratch.

Working with services like this can save both time and money, especially for those looking to move quickly through the design and permitting stages.

Example: A 750 sq. ft. Detached ADU

Here’s what a typical mid-size detached ADU might include in its cost breakdown:

• Foundation & Framing: $80,000–$100,000

• Plumbing, Electrical, HVAC: $40,000–$60,000

• Insulation, Drywall, Paint: $25,000–$35,000

• Cabinets, Flooring, Tile, Fixtures: $35,000–$45,000

• Exterior Siding, Roofing, Windows, Doors: $35,000–$50,000

• Permits, Design, and Engineering: $10,000–$20,000

Total Estimated Cost: $225,000–$310,000

Additional Costs to Budget For

Even with a well-planned project, a few common expenses fall outside the construction contract:

• Site Survey & Engineering: $2,500–$7,500

• Permitting Fees: $1,000–$3,500

• Septic Upgrade (if needed): $20,000–$50,000

• Driveway or Landscaping Work: $5,000–$15,000

• Furniture & Appliances: $10,000–$25,000

Accounting for these early helps you plan a more accurate all-in budget.

How to Save on Your ADU Project

• Start with a permit-ready plan. Working from a pre-designed plan through MassADUPlans.com reduces architectural costs and speeds up permitting.

• Keep the design simple. Every corner, dormer, and bump-out adds cost.

• Plan ahead. Securing materials and builder availability early avoids seasonal price increases.

• Choose experienced local builders. Contractors familiar with your town’s requirements can prevent costly delays.

The Bottom Line

In 2025, most homeowners in Massachusetts can expect to spend $225,000–$350,000 for a high-quality ADU that meets state standards.While that’s a significant investment, the payoff can be huge — from long-term rental income to increased property value and multi-generational flexibility.

Working with an experienced builder and leveraging permit-ready design services like MassADUPlans.com can make the process smoother, faster, and more cost-effective.

Ready to Explore Your ADU Options?

If you’re considering adding an ADU on Cape Cod, Martha’s Vineyard, or the South Shore, our team at McElhinney Construction can guide you through every step — from site feasibility to construction.

We’ll help you understand your property’s potential, coordinate with designers, and build an ADU that adds lasting value.

Let’s bring your ADU project to life — the right way.

Talk to our team