So you’ve heard the buzz about Accessory Dwelling Units, or maybe you read Part 1 and are now wondering: “Okay, but what does it actually cost? And will it really pay off in my neighborhood?”
Let’s break it down without the hype, just the numbers and local context.
What It Actually Costs
In Boston, you’ve basically got two main paths: convert space you already have (like a basement or garage) or build something new in your yard.
Conversions turning part of your existing home into an ADU typically run $75,000 to $100,000. Detached ADUs, the kind you see tucked behind a Roslindale triple-decker or Dorchester colonial, usually land closer to $250,000 to $350,000 once you factor in plumbing, utilities, and permits.
That’s the range of city and state data points right now. Keep in mind: smaller, simpler projects can land lower, but it’s easy to overshoot if you start customizing finishes or face tricky site work.
For planning purposes, most Boston homeowners should expect a price in the $200K to $300K range for a straightforward detached unit.
What You Could Earn
Let’s talk about the other side of the equation: income.
Rents in Roslindale and Dorchester for small apartments hover around $2,000 to $2,400 per month for one-bedrooms and $2,700 to $3,100 for two-bedrooms. That means a well-designed ADU could comfortably bring in $2,000 to $3,000 each month, depending on size, finishes, and proximity to transit.
For example, imagine you spend about $250,000 to build a detached unit that rents for $2,500 a month. That’s $30,000 in gross income per year. After accounting for expenses like maintenance, taxes, and insurance, you might net around $21,000 per year, which puts your simple payback at roughly twelve years. Not a quick flip, but a stable, income-producing asset that grows in value over time.
Design Traps to Avoid
Even though ADUs are smaller projects, they can go sideways if you skip the details. Here are the five traps I see most often:
- Underestimating site work – utility connections and foundation work can eat your budget alive.
- Overbuilding for the neighborhood – marble countertops won’t raise rent if the market caps at $2,500.
- Skipping zoning research – each Boston neighborhood still has its own overlay rules.
- Ignoring layout flow – separate entrances and good soundproofing are non-negotiable for happy tenants.
- Designing for yourself instead of the market – match the layout to what local renters actually want.
The Path: From Idea to Income
Here’s the high-level roadmap:
- Check feasibility: confirm your lot, zoning, and utilities can support an ADU.
- Design concept: work with a designer or architect to create a plan that fits your lot and your goals.
- Budget and finance: include soft costs such as permits, design, and contingencies. Boston even offers ADU support programs through the Boston Home Center
- Permitting: submit your plans, adjust if needed, and get the green light.
- Build: manage your contractor closely; unexpected delays can happen.
- Lease: photograph, list, and rent. Aim for the sweet spot on price to minimize vacancy.
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Review: after a year, revisit your numbers to see what worked and what could improve.
Final Thoughts
ADUs aren’t a fad. They’re a smart way to future-proof your property and create flexible, income-producing space without buying new land. Whether you’re helping a parent age in place, planning for rental income, or laying the foundation for long-term wealth, the math can make sense if you plan carefully.
The key is treating your ADU as the investment it truly is. Not a project for someday, but a financial strategy for right now.
Ready to see if your home qualifies for an ADU? Reach out, and we’ll walk you through the first steps.