Costs & Budgeting · July 21, 2026 · 5 min read
What Actually Drives the Cost of an ADU
Cost per square foot is a comforting number and a misleading one. An ADU budget is shaped far more by site conditions and utilities than by the footprint of the building.
The factors that move the number most
In rough order of impact on a typical Rhode Island or Massachusetts project:
- Septic or sewer work, including expansions and new systems
- Utility runs and service upgrades from the main home
- Site conditions — ledge, slope, drainage, access for equipment
- Foundation type
- Unit size and the number of bathrooms and kitchens
- Finish level and selections
- Permitting and engineering requirements specific to the town
How a range becomes a real number
Early in planning you get a preliminary range — wide on purpose, because the unknowns are still unknown. As design develops and site information comes in, that range narrows. Once drawings and scope are complete, you get detailed line-by-line pricing rather than an allowance-heavy lump sum.
Before you sign a construction contract, we review that pricing together: what is included, what is excluded, what is an allowance, and where overhead and profit sit.
Where value engineering fits
If the number needs to come down, the right time to adjust is before construction. That may mean simplifying a roofline, shifting a finish allowance, or rethinking a layout — decisions made deliberately, on paper, instead of mid-build.
Wondering what your property could support?
Tell us about the property and what you’re hoping to accomplish in Rhode Island or Massachusetts. We’ll help you understand what’s realistic.
Prefer to talk? Call or text 401-388-3889.