Low-threshold, defined by what it skips
A low-threshold shower base sits directly on the existing floor like a standard shower pan, but its outer wall is shorter than a typical tub wall, so the step to get in is smaller. It doesn't require touching the framing beneath the floor, which is why it's the easier upgrade of the two.
Most low-threshold prefab pans install the same way a standard shower base does: set on the subfloor, connect to the existing or relocated drain, seal the edges. That's part of why a straightforward prefab conversion lands at the low end of the $2,000 to $6,000 range for prefab surrounds.[1]
Curbless, defined by the framing it touches
A curbless shower has no raised edge at all; the shower floor is flush with the rest of the bathroom floor. To make that work, the subfloor inside the shower footprint has to be recessed, commonly by notching or sistering the floor joists, or framing a lowered section, so a sloped mortar bed and drain assembly have room to sit below the finished floor line.[2]
One manufacturer's recessing bracket system for a point-drain shower calls for an 11/16-inch recess in the subfloor itself, on joists spaced no more than 19.2 inches apart, without notching the joists at all. Other curbless methods that use a linear drain instead of a point drain recess the floor by a larger amount to fit the drain body and the slope down to it.[2]
Cost and timeline follow the framing
Curbless showers run $8,000 to $15,000 in national data, well above the $2,000 to $6,000 for a standard prefab conversion, because the structural work happens before waterproofing or tile can even start.[1]
That structural step is also why a curbless conversion is harder to reverse or retrofit into an existing floor without opening the ceiling below, which matters if you live in a condo or a home with finished space underneath the bathroom.
What to check before you commit to curbless
Ask what's below the bathroom floor. A slab-on-grade bathroom has no joists to notch or sister, so a curbless build there usually means cutting into the slab itself, a bigger and dustier job than working with a wood-framed floor.
Ask how the transition to the rest of the bathroom floor is finished. A curbless shower still needs a slope down to the drain, and that slope has to blend into the surrounding floor without creating a trip hazard or sending water out past the shower's footprint.
Ask which drain type the plan uses and why. A point drain, centered under the shower, is a smaller structural change than a linear drain running the width of an entry, since a linear drain typically needs a wider recessed channel to sit in.