Quick answer: ADU costs in Rhode Island vary by type. As planning ranges, a basement conversion runs about $80,000 to $160,000, a garage conversion $90,000 to $180,000, an attached in-law suite inside the existing footprint $100,000 to $200,000, and a detached backyard cottage $225,000 to $400,000 or more. This guide builds on our ADU Rules in Rhode Island: 2026 Guide, which covers the state's zoning reform and what it changed. Here, the focus is what to expect on cost and schedule once you've confirmed an ADU is legally possible on your property.
Why Cost Varies So Much by ADU Type
Garage conversions tend to be among the most cost-effective ADU types because the structural shell already exists. The work is primarily insulation, framing interior walls, adding plumbing and electrical, and finishing the interior. Attached in-law suites convert existing square footage (often part of a first floor or an attached wing) into a self-contained unit, which avoids new foundation work but still requires a full kitchen and bath build-out. Basement conversions depend heavily on existing ceiling height and require particular attention to moisture management and code-required egress, which can add cost if those aren't already adequate. Detached backyard cottages involve the most construction, including a new foundation, framing, and full systems, but offer the most design flexibility and privacy, and are usually the highest-cost option of the four.
ADU Price Ranges by Type
Basement conversion: $80,000 to $160,000.
Garage conversion: $90,000 to $180,000.
Attached in-law suite inside the existing footprint: $100,000 to $200,000.
Detached backyard cottage: $225,000 to $400,000 or more.
These are planning ranges as of October 2026, not quotes. Where a project lands depends on size, finish level, site conditions, utility connections, and what your town requires. A real number requires seeing the property.
What Affects Timeline
ADU timelines in Rhode Island are shaped by three phases: design, permitting, and construction. As covered in our ADU rules guide, permitting timelines vary meaningfully by municipality. Some East Bay towns move ADU applications through quickly, while others have backlogs that add real time to the schedule before construction can even begin. If your property is in a designated historic district, that adds a separate local historic district commission approval to the timeline as well (see our guide to renovating in a Rhode Island historic district for what that process involves). A garage conversion can often be completed in a few months. A new detached structure typically takes longer once you factor in design, permitting, and construction.
Budgeting for the Unexpected
Garage conversions and basement conversions in particular can surface conditions that aren't visible until work begins: outdated wiring in an older garage, moisture issues in a basement that hasn't been finished before, foundation conditions that need addressing. Building in contingency room for this kind of discovery, rather than budgeting to the exact estimate, tends to make the process less stressful if something does come up.
How Toensing Construction Approaches ADU Projects
Because feasibility, cost, and timeline are all so specific to a given property and municipality, Toensing Construction starts every ADU inquiry with an on-site consultation to understand what your lot allows, what type of ADU makes sense for your goals, and what a realistic budget and schedule looks like before you commit to a design. We coordinate with local architects and designers where needed and manage the project from foundation to finish. See our ADU construction page for more.
Frequently Asked Questions
Which ADU type is generally the fastest to build?
Garage conversions are typically the quickest, since the structural shell already exists and there's no new foundation work involved, though the specific timeline still depends on your municipality's permitting schedule.
Does the state ADU law mean I don't need a permit?
No. The reform requires municipalities to allow ADUs on single-family lots and removed several restrictive local barriers, but building permits and inspections are still required for all ADU construction, and local rules on size, height, and design standards still apply.
Can I get a real cost estimate before committing to a design?
Yes. That's the purpose of the on-site consultation. Because ADU costs vary so much by type and by what your specific property allows, a real number requires seeing the property first.
If you're considering an ADU anywhere in Rhode Island's East Bay or beyond, reach out to Toensing Construction to talk through what's feasible on your property.