San Diego's ADU boom has produced a lot of shared wall living arrangements that did not exist on these lots before, whether that is a converted garage JADU sitting under a bedroom, a detached unit close enough to carry sound, or a multigenerational suite meant to feel private even though it shares structure with the main house. Flooring plays a bigger role in how that arrangement actually feels day to day than most homeowners plan for, and IIC and STC ratings are the two numbers that predict it.
The problem is these two ratings get used interchangeably in casual conversation, and they measure completely different things. Choosing flooring based on the wrong one is a common reason a new ADU feels noisier than expected within the first few weeks of occupancy.
What STC Actually Measures
STC, or Sound Transmission Class, measures how well a floor or wall assembly blocks airborne noise like conversation, television audio, or music moving from one space to another. A higher STC number means less of that airborne sound gets through the assembly, which matters most when you are trying to keep a phone call or TV show in one unit from being audible in another.
STC is the rating most people think of by default when they hear the word soundproofing, largely because it maps to the kind of noise people notice complaining about most. But in a shared wall or stacked ADU layout, it is often not the noise causing the actual daily friction.
What IIC Actually Measures
IIC, or Impact Insulation Class, measures how well a floor blocks impact noise, meaning footsteps, dropped objects, dragged furniture, and anything else that physically strikes the floor surface. This is a completely separate transmission path from airborne sound and requires a different kind of underlayment and flooring system to control.
In stacked or shared structure ADU layouts, impact noise is almost always the bigger daily irritant, since footsteps overhead or a dropped item register far more sharply through a shared floor than a muffled conversation does. A flooring system can have a perfectly respectable STC rating and still transmit footsteps clearly if the IIC rating was never part of the selection.
Why This Distinction Matters for Your ADU Layout
A detached ADU with its own foundation, like most backyard cottages being built across San Diego right now, has less shared structure to worry about and lower stakes on both ratings. A garage conversion JADU or an ADU built with a bedroom directly above or adjacent to shared framing is a different situation entirely, and that is where IIC deserves the primary focus during flooring selection.
This is also where underlayment choice matters as much as the flooring material itself. Two identical laminate planks installed with different underlayment can produce meaningfully different IIC results, which is why the underlayment spec should be treated as part of the flooring decision rather than an afterthought added at checkout.
Choosing Flooring With the Right Rating in Mind
For any ADU with shared structure or a stacked layout, ask for the IIC rating of the full flooring assembly, not just the plank, since the number changes significantly based on what sits underneath it. A laminate or engineered hardwood paired with a high performance acoustic underlayment can perform dramatically better than the same plank installed with a basic foam pad.
This is not a decision to make from a spec sheet alone, since the right assembly depends on your specific ADU layout, what sits above or beside it, and how the space will actually be used day to day.
America's Best Flooring can walk you through IIC rated flooring assemblies built specifically for shared structure and stacked ADU layouts across Carmel Valley, La Jolla, Del Mar, Poway, Coronado, Rancho Santa Fe, Solana Beach, East Lake, Chula Vista, La Mesa, Encinitas, Carlsbad, Cardiff, San Marcos, Escondido, Santee, El Cajon, Rancho San Diego, and Ramona, CA. Visit San Diego, CA to compare underlayment options in person, browse our laminate flooring and hardwood flooring lines built for sound performance, or contact us to get your specific layout evaluated before you buy.
