Detached, Attached, or Internal? Choosing the Right Type of ADU for Your Property

Massachusetts law allows an ADU in almost any form — detached, attached, or carved out of space you already own. The right choice comes down to your lot, your budget, and who's going to live there. Here's how the options compare after the state's by-right ADU law leveled the playing field.

Internal ADUs: Basement & Attic Conversions

Best for: budget-conscious owners, rental income. Cost: $80K–$150K. Timeline: 2–4 months.

Converting existing space is the cheapest path to a legal second unit. You'll need 6'8"+ finished ceiling height, proper egress, and fire separation between units. The trade-off is light and ceiling height — great for tenants, tighter for long-term family living.

Garage Conversions

Best for: speed and value. Cost: $120K–$220K. Timeline: 3–5 months.

The structure, roof, and slab already exist — you're buying insulation, systems, and finishes. Read our dedicated garage conversion guide for the slab, insulation, and utility details.

Attached Additions

Best for: multigenerational living. Cost: $180K–$350K. Timeline: 4–7 months.

An addition ADU shares a wall (and often mechanicals) with your house while keeping a separate entrance. It's the natural evolution of the in-law suite — see how families are using them in our multigenerational ADU article.

Detached Backyard Cottages

Best for: maximum privacy and rental value. Cost: $250K–$500K+. Timeline: 5–9 months.

A standalone cottage commands the highest rent and adds the most appraised value, but it's full new construction. Setbacks and lot coverage determine placement; utility runs drive cost.

The Size Math

Whatever the type, the cap is 900 sq ft or half your home's gross floor area, whichever is smaller. A 900 sq ft ADU comfortably fits a one-bedroom with open living space, or a compact two-bedroom at around 750–900 sq ft.

Not sure which fits your lot? Our ADU team will tell you in one site visit.