At first glance, converting your garage into an Accessory Dwelling Unit (ADU) seems like an obvious win. You’re already starting with a structure, right? Just add some windows, install a bathroom, and voilà—instant rental income or in-law suite.
The reality is a bit more complicated.
At Dwelly, we’re all about making ADUs accessible and streamlined for homeowners across New England. But we also believe in setting clear expectations so you can make informed, cost-effective decisions.
If you’re considering a garage conversion, here’s what you need to know before you get started.
Why Garage Conversions Are Popular
Garage-to-ADU conversions are appealing because they seem like a shortcut:
- The structure already exists
- The footprint is set
- You may already have power running to the garage
But garages are designed for cars—not people. Upgrading them into habitable spaces requires thoughtful planning, and sometimes, starting fresh is actually the better path.
Slab and Foundation: It’s Not Just About Walls
Most garages are built on a basic concrete slab, which typically isn’t insulated or designed to prevent moisture infiltration. For a living space, this can lead to long-term comfort and durability issues.
Common slab issues:
- No vapor barrier beneath the concrete
- Insufficient insulation (or none at all)
- Inadequate structural support for new loads
Potential fixes:
- Remove and replace the slab with insulation below
- Pour a new insulated slab over the existing one
- Frame an insulated floor above the existing slab (though this reduces interior ceiling height)
Wall Framing: Insulation and Structural Concerns
Garages usually have minimal wall framing—sometimes just enough to support a garage door and hold the roof up. The walls often lack insulation, sheathing, or proper weather barriers.
What you might need to do:
- Upgrade the framing to meet residential code
- Add insulation (to meet new energy codes like IECC 2021 or local stretch codes)
- Replace or add weather-resistant barriers and interior drywall
Roof Framing: The Insulation Challenge No One Talks About
Here’s where many garage conversions run into a deal-breaker: roof framing.
Most garage roofs are built with 2×6 rafters, which simply aren’t deep enough to hold the amount of insulation required for a habitable space, especially in cold climates like New England.
Why this matters:
- Building codes often require R-38 insulation or higher for roofs. That usually means you need at least 10-12 inches of space for insulation and ventilation.
- With shallow rafters, you can’t meet modern energy codes without reframing the roof entirely.
Solutions:
- Reframe the roof with 2x10s or 2x12s to allow for proper insulation depth
- Use exterior insulation strategies (like adding rigid foam above the roof deck)
- Consider building new if reframing costs get too high
Utilities: Plumbing and Power Upgrades
Most garages don’t have plumbing, and the electrical systems are typically limited to a few outlets and a light fixture. Upgrading your garage to an ADU usually requires:
- Running new water supply and sewer lines
- Adding a new electrical subpanel or upgrading service to the house
- Installing HVAC systems for heating, cooling, and ventilation
These infrastructure upgrades can significantly impact cost and timeline.
Permitting and Zoning: Don’t Skip This Step
Before you dive into plans, make sure to check your local zoning laws. In Massachusetts and Rhode Island, new ADU-friendly legislation has made conversions easier—but not every garage is automatically eligible.
Considerations:
- Setbacks and lot coverage limits
- Parking requirements
- Historic district regulations (if applicable)
- Egress windows and life safety codes
Your city or town may also require special inspections for existing structures before approving a conversion.
When to Consider a Rebuild Instead
Sometimes the best choice is to start from scratch. If your garage has:
- Poor structural integrity
- Significant slab issues
- Undersized roof framing that requires major modifications
- Outdated materials or code violations
Then building new might actually be more cost-effective and sustainable. A new ADU can be designed specifically for energy efficiency, comfort, and long-term durability—without trying to retrofit an old structure.
At Dwelly, we offer pre-designed, permit-ready ADU plans that can help you streamline this process.
The Bottom Line: Know Before You Build
Garage conversions can be a great solution—but they’re not always the easiest or cheapest route. Whether you retrofit or rebuild, the key is to:
- Understand the true scope of work
- Plan for upgrades beyond finishes
- Work with experts who know local codes
At Dwelly, we’re here to help you navigate the process from start to finish. Whether you’re converting an existing garage or starting fresh with one of our pre-designed ADUs, we’ll help you make the right decision for your property, your budget, and your goals.
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