Typical planning ranges
- Garage, basement, or attic conversion: often roughly $120,000–$220,000.
- Attached ADU (addition to the home): often roughly $200,000–$350,000.
- Detached ADU (new standalone cottage): frequently $250,000–$450,000 and up.
- These are broad planning figures, not quotes.
What drives the cost
- Utilities: new water, sewer or septic, and electrical service can be significant on a detached unit.
- Foundation and site work: slope, ledge, and access matter.
- Size and finish level: kitchens and bathrooms are the most expensive square feet in any ADU.
- Local review: setbacks, parking, and design requirements vary by town.
One team from plans to keys
- We handle feasibility, architectural plans, permitting, and construction in-house, so zoning and budget questions get answered before you commit to a design.
Ranges are general planning estimates for the Greater Boston and Massachusetts market. Every project is priced individually — contact us for a line-by-line number.