The Hidden Costs of Building an ADU in Massachusetts
Accessory Dwelling Units (ADUs) are an exciting way for Massachusetts homeowners to add value, create rental income, or support multigenerational living. While most people focus on the headline cost of construction, there are often hidden expenses that can catch homeowners off guard. Budgeting for these in advance will help you avoid surprises and keep your project on track.
1. Permitting & Fees
Building Permits: Every town has its own fee structure, often tied to project size or cost.
Impact Fees: Some municipalities may charge for added utility demand or infrastructure impact.
Waste Disposal & Dumpster Rentals: Often overlooked but necessary during construction.
💡 Tip: Ask your town’s building department for a complete fee schedule before starting design.
2. Design & Professional Services
Architectural Plans: Massachusetts requires stamped plans from a licensed architect or engineer.
Surveying: A property survey may be needed to confirm lot lines, setbacks, or sewer tie-in locations.
Engineering Studies: Structural, soil, or grading studies may be required depending on your site.
💡 Tip: Budget 10–15% of your total project cost for professional services.
3. Utility Upgrades
Electrical Service: Adding an ADU often requires panel upgrades or new wiring.
Plumbing & Sewer: Tie-ins to municipal water/sewer can be costly, especially if pumps are needed.
Separate Meters: If you want independent billing for tenants, you’ll need new gas, water, or electric meters installed.
💡 Tip: Factor in trenching, pump systems, or long tie-ins if your ADU is detached from the main home.
4. Site Work & Infrastructure
Excavation & Grading: Leveling a lot or digging for foundations can quickly add thousands.
Driveway & Parking: Some towns require additional parking spaces.
Drainage Improvements: Stormwater management may be required to meet code.
💡 Tip: Get a contractor’s estimate for site prep early—it’s one of the biggest “wild card” costs.
5. Carrying Costs During Construction
Temporary Housing: If major renovations displace you during construction.
Higher Insurance Premiums: Coverage during construction often costs more.
Extended Timelines: Permitting or supply delays mean longer periods of carrying mortgage + extra costs.
💡 Tip: Add a 10–15% contingency to your budget to account for delays.
6. After It’s Built
Property Taxes: Your home will be reassessed and taxed on the higher value.
Maintenance & Repairs: Two kitchens, two bathrooms, two sets of appliances = higher upkeep.
Tenant Turnover Costs: If renting, expect cleaning, marketing, and minor repairs between tenants.
💡 Tip: Treat your ADU like a small business—budget for long-term operating costs, not just construction.
Final Thoughts
Building an ADU in Massachusetts is a smart investment, but only if you prepare for the true cost of ownership. Beyond construction, you’ll need to account for fees, professional services, utilities, site work, and ongoing expenses.
By budgeting realistically—including a 10–20% contingency fund—you’ll set yourself up for a smoother building process and ensure your ADU delivers the financial and lifestyle benefits you’re aiming for.
📞 Ready to explore your options?
Call us at (617) 797-9167 or email leah@mass-adu.com to set up a free site consultation.
