Types of ADUs
Detached, attached, or internal — each type has different trade-offs for cost, timeline, privacy, and livability.
Under Massachusetts’ Affordable Homes Act, accessory dwelling units up to 900 sq ft are allowed by right on most single-family lots. You can build a detached cottage in the backyard, add an attached addition to your home, or convert an existing basement, attic, or garage. Each approach has different costs, timelines, and trade-offs — here’s how they compare.
Detached ADU
Standalone cottage or backyard home
A separate structure in your yard with its own entrance and full independence from the main home. Detached ADUs offer the most privacy and flexibility — ideal for rental income or an aging parent who values their own space.
Pros
- Maximum privacy for both households
- Best rental income potential
- No disruption to main home during construction
- Can match or complement your home's style
Cons
- Highest cost (new foundation, all new utilities)
- Requires adequate yard space and setbacks
- Longer construction timeline
Size
400–900 sq ft
Timeline
4–6 months
Relative Cost
Highest
Best for: Rental income, aging parents who want independence, guest house
Attached ADU
Addition to your existing home
Built onto the side, back, or above your existing house with a shared wall. Attached ADUs balance cost savings with a degree of separation — a private living space that's still just steps away.
Pros
- Lower cost than detached (shared wall, nearby utilities)
- No yard space consumed
- Can share HVAC and plumbing runs
- Adds directly to your home's footprint and value
Cons
- Construction may disrupt daily life
- Less privacy (shared wall)
- May require fire-rated wall separation
Size
300–900 sq ft
Timeline
3–5 months
Relative Cost
Moderate
Best for: Aging parent who wants proximity, expanding home value, properties with small yards
Internal ADU
Basement, attic, or garage conversion
Converting existing space within or attached to your home — a finished basement, attic buildout, or garage conversion. Internal ADUs are the fastest and most affordable path to a second dwelling unit.
Pros
- Lowest cost (existing structure and envelope)
- Fastest construction timeline
- No change to your home's exterior footprint
- Often the simplest permitting process
Cons
- Limited by existing space dimensions
- May need egress windows or additional exits
- Less natural light, potential ceiling height constraints
Size
300–700 sq ft
Timeline
2–4 months
Relative Cost
Lowest
Best for: Budget-conscious projects, basement apartments, garage conversions, home offices
Side-by-Side Comparison
| Detached | Attached | Internal | |
|---|---|---|---|
| Typical Size | 400–900 sq ft | 300–900 sq ft | 300–700 sq ft |
| Privacy Level | High | Moderate | Low–Moderate |
| Construction Timeline | 4–6 months | 3–5 months | 2–4 months |
| Relative Cost | Highest | Moderate | Lowest |
| Permit Complexity | Standard | Standard | Simplest |
| Foundation Required? | Yes (new) | Partial (extension) | No (existing) |
| Separate Entrance? | Yes | Yes | Yes (required by code) |
| Energy Code Impact | Full compliance (new build) | Full compliance (addition) | Varies by scope |
| Best For | Rental income, guest house | Aging parents, added value | Budget projects, conversions |
Which Type Is Right for You?
“I want rental income”
A detached ADU offers the most privacy and commands the highest rents. Tenants get their own yard space, entrance, and address-like independence.
Our pick: Detached ADU
Talk to us →“I’m on a tighter budget”
An internal conversion — basement, attic, or garage — reuses your existing structure and keeps costs down. It’s also the fastest to complete.
Our pick: Internal ADU
Talk to us →“My parent is moving in”
An attached addition keeps your family close while providing a private, accessible living space with its own entrance. Just steps away, not miles.
Our pick: Attached ADU
Talk to us →Not Sure Which Type Fits Your Property?
We'll visit your lot, assess your options, and recommend the best approach. Free, no obligation.