Attached ADUs in Boston
An attached ADU is a fully permitted second unit that shares a wall with your main house, typically built as an addition, with its own kitchen, bathroom, and private entrance. It's the middle-ground ADU: more affordable than a standalone detached cottage and a better fit for tighter lots, while still adding a complete independent home. Attached ADUs are allowed by right under Massachusetts' 2025 ADU law, and most in the Boston area run $150,000–$300,000+.
Attached ADU: Key Facts
- Type: Second unit connected to your main house (shares a wall)
- Built as: An addition, or a conversion of existing living space
- Max size: Generally up to 900 sq ft
- Cost range: $150,000–$300,000+
- Legal status: Allowed by right statewide since Feb 2, 2025
- Best for: Tighter lots, lower cost than detached, staying connected to family
What Is an Attached ADU?
An attached ADU is a self-contained, fully permitted second home that shares at least one wall with your main house, with its own kitchen, bathroom, and private entrance. It's created one of two ways: by building an addition onto your home, or by converting existing main-house living space into a separate unit.
(Converting an existing garage or basement is its own path — see our garage conversion and basement ADU guides. This page covers additions and conversions of main living space.)
A completed attached ADU typically includes:
- A shared wall with the main house, plus its own private entrance
- A full kitchen and bathroom
- Defined living and sleeping areas
- Code-required fire and sound separation between the units
- Its own utility connections, shared or separately metered
Homeowners across the Boston area build them for:
- Housing a parent or adult child while staying physically connected
- Long-term rental income
- Caregiving that's easier under one connected roof
- Adding a permitted unit when the lot is too tight for a detached build
Why Choose an Attached ADU
For many homes, an attached ADU is the most practical path to a full second unit.
Works on Tighter Lots
When required setbacks leave no room for a freestanding cottage, an attached unit added to the house often still fits.
Lower Cost Than Detached
You're not pouring a full separate foundation or building a complete standalone shell, so an attached ADU usually costs less than a ground-up detached build.
Stays Connected
A shared wall keeps family physically close, ideal for caregiving or for parents who want help nearby without a separate building.
Integrates With Your Home
If done well, an attached ADU reads as part of the house, preserving your curb appeal. It's the middle option between a conversion and a detached cottage, in both cost and independence.
Attached vs. Detached ADU
Both add a full second unit, but they suit different lots and goals. Here's how they compare.
| Factor | Attached ADU | Detached ADU |
|---|---|---|
| Typical cost | $150,000–$300,000+ | $200,000–$350,000+ |
| Privacy | Shares a wall with the main home | Highest, fully separate building |
| Design freedom | Constrained by the existing house | Most, standalone blank slate |
| Lot needs | Works on tighter lots | Requires yard space and setbacks |
| Timeline | Shorter than detached | Longest, full ground-up build |
| Best when | Your lot is tight or you want lower cost | You have the lot room and want privacy |
Want maximum privacy and have the yard space? See our detached ADU guide →.
Comparing all your options, not just these two? Our in-law suite guide lays out all four ADU types side by side.
Two Ways to Build an Attached ADU
An attached ADU is created one of two ways, and the right choice depends on your home's layout and your budget.
- An addition (bump-out): new square footage appended to the house. It gives you the most control over size and layout, but it costs more and needs both lot room and a new section of foundation.
- Converting existing living space: turning an underused wing, an upper floor, or an old in-law area into a separate unit by adding a kitchen and a private entrance. Usually cheaper since there's no new footprint, but you're working within your existing layout.
Pro tip from our ADU builders in Boston: Look hard at converting existing space before you assume you need an addition. An underused wing or a full second floor can often become a permitted unit for far less than building out, the kitchen and a separate entrance are usually the main additions, not a whole new foundation.
How Much Does an Attached ADU Cost in Massachusetts?
Most attached ADUs in the Boston area fall between $150,000 and $300,000+, with conversions of existing space at the low end and additions at the higher end. The single biggest factor is which of those two routes you take.
The biggest cost factors:
- Addition vs. conversion: building new square footage costs more than converting space you already have.
- Tying into the existing structure: matching the foundation, roofline, and walls where the new unit meets the house.
- Fire and sound separation: the code-required rated wall and floor/ceiling assembly between two dwelling units.
- Utility separation: sharing existing electric and water versus running and separately metering new service.
- Finish level: basic rental-grade versus higher-end finishes.
Where an Attached ADU Budget Goes
Every project differs, but here's roughly how a mid-range attached build tends to break down. Treat these as illustrative ranges, not a quote.
| Where the budget goes | Typical range |
|---|---|
| Foundation and structural tie-in | $25,000 - $50,000 |
| Addition framing or conversion build-out | $45,000 - $85,000 |
| Fire/sound separation and utilities | $20,000 - $40,000 |
| Interior (kitchen, bath, HVAC, finishes) | $45,000 - $85,000 |
| Design and permits | $15,000 - $28,000 |
A feasibility assessment turns that range into a real number for your specific home.
Attached-Specific Considerations
A few things make or break an attached ADU, and they're different from any other type:
- Fire separation: a fire-rated wall and floor/ceiling assembly between the two units, as the Massachusetts building code requires.
- Sound separation: insulated party walls and floors are what actually make shared-wall living work day to day.
- Shared vs. separate utilities: whether electric and water are shared or separately metered, which matters most if you'll rent the unit.
- Structural tie-in: connecting the new unit to the existing house's foundation, walls, and roofline.
- A private, separate entrance: required to permit it as an independent unit.
- Matching the exterior: so an addition reads as part of the home, not a bolt-on.
- Setbacks: additions that extend the footprint still have to meet your town's distance-from-property-line rules.
Pro tip from Horizon ADU: Decide on separate utility metering up front. Running and metering the unit's own electric and water is a straightforward choice during the build but an expensive retrofit later, and it's worth doing if you'll ever rent the ADU, since it keeps the tenant's usage cleanly separate from yours.
Pros and Cons of an Attached ADU
An attached ADU is the practical middle option, and it isn't right for every home. Here's an honest look at both sides.
Pros
- Works on tighter lots where a detached unit won't fit.
- Lower cost than a ground-up detached build.
- Keeps family physically connected, ideal for caregiving.
- Often faster than a detached build, especially as a conversion.
Cons
- Less privacy than detached, since it shares a wall.
- Constrained by your existing home's layout and structure.
- Fire and sound separation add cost a conversion otherwise wouldn't.
- An addition still needs setback room and a new foundation section.
Attached ADU FAQs
What makes an ADU "attached"?
An attached ADU shares at least one wall with your main house, rather than standing on its own like a detached cottage. It's built either as an addition or by converting existing living space.
Is it cheaper to add on or convert existing space?
Converting existing space is usually cheaper, since there's no new foundation or footprint — the main additions are a kitchen and a private entrance. An addition costs more but gives you more control over size and layout.
Do the two units need fire separation?
Yes. The Massachusetts building code requires a fire-rated wall and floor/ceiling assembly between two dwelling units, which we build in as part of the project.
Can utilities be separate or separately metered?
Yes. You can share existing utilities or run and separately meter the unit's own electric and water. Separate metering is worth deciding on up front, especially if you'll rent it.
Does an attached ADU need its own entrance?
Yes. A private, code-compliant entrance is required to permit it as an independent unit, and we design it to fit your home's exterior.
How long does an attached ADU take
It's typically faster than a detached build — a conversion especially — though the exact timeline depends on permitting, whether it's an addition or conversion, and scope.
Can I rent it out?
Yes. Most Boston-area municipalities allow permitted ADUs as long-term rentals, while short-term rentals generally are not permitted under the state ADU rules.