We get a lot of questions about accessory dwelling units (ADUs), so here are the basics.
Yes, you can probably build one
Rhode Island’s ADU law, R.I. Gen. Laws § 45-24-73, as amended in 2024, requires every municipality to allow one ADU per lot by right on any residential lot of 20,000 square feet or more, or anywhere the ADU fits within the existing footprint of the house or an existing accessory structure (a garage, barn, etc.). Towns can no longer simply say no.
The unit doesn’t have to be for family. The statute prohibits towns from restricting tenants based on family relationship or age, so you can rent it to anyone, as long as it’s a long-term rental. Short-term and transient rentals (Airbnb, VRBO, etc.) are not allowed.
Size limits
Under § 45-24-73(b), a town may cap ADU size, but the cap must allow at least 900 sf (or 60% of the main house, whichever is less) for a studio or one-bedroom, and 1,200 sf (or 60%, whichever is less) for a two-bedroom. Middletown’s ordinance is a good, clean summary of how most towns are applying the state rules:
- Studio or one-bedroom ADU: 900 sf max, or 60% of the floor area of the main house, whichever is less
- Two-bedroom ADU: 1,200 sf max, or 60% of the main house, whichever is less
- No more than two bedrooms
- A detached ADU follows the same setbacks as any accessory structure in that zone, and a new one can’t sit in the front yard
- One off-street parking space
- No short-term rentals
Every town is a little different
What’s above is a general overview. Each municipality has written its own ADU ordinance, and the details (parking, setbacks, application process, fees) vary from town to town. We’ll check your town’s specific requirements as part of the project.
What you need for a permit
Plan on a survey, a site plan, and architectural plans. We prepare all three in house.
The real question: water and sewer
Zoning is rarely what stops an ADU. Utilities are. If you’re on town sewer and water, you’re in good shape. If you’re on a septic system, we need to check the capacity of your existing OWTS, because septic systems in Rhode Island are sized by bedroom count. The state law specifically allows towns to require septic upgrades where needed to address capacity for the ADU, and some towns (Warwick, for example) formally require a Residential OWTS Suitability Determination from RIDEM before they’ll permit an ADU on an unsewered lot.
In most cases, there is no spare capacity. That leaves two options:
- Give up a bedroom in the main house. Often the simplest fix is converting an existing bedroom so it no longer counts as one, for example by removing the door and replacing it with a 4-foot cased opening. Your total bedroom count stays the same, and the existing system still works.
- Increase the septic capacity. There are many ways to do this, from an expanded leachfield to an advanced treatment system, but feasibility always comes down to the soils.
Start with a soils test
If you’re on septic and serious about an ADU, a soil evaluation is often the smartest first step. It tells us quickly whether an upgrade is realistic, and that answer drives everything else: layout, budget, and whether the bedroom swap is the better path.
Thinking about an ADU? Give us a call at 401-683-6630. We’ll help you figure out what’s feasible on your property before you spend money on design.