Sound between units: what the code requires, and the noise it ignores
The BC Building Code gives you two ways to comply and one large exclusion. Footstep noise, the complaint we hear most from people living in new multi-unit buildings, is not covered at all.
In short, The BC Building Code 2024 requires a dwelling unit to be separated from other spaces where noise may be generated by either an ASTC rating of not less than 47, counting the separating assembly plus adjoining constructions, or an STC rating of not less than 50 for the separating assembly with flanking construction conforming to Article 9.11.1.4. Part 3 and Part 9 set identical numbers. Construction separating a dwelling unit from an elevator shaft or refuse chute requires STC not less than 55. Secondary suites in Part 9 may use a reduced standard of STC 43 or ASTC 40, or a prescriptive assembly. The code contains no requirements for impact noise transmission.
Ask anyone who has lived in a newer multi-unit building what bothers them about it, and you will rarely hear about conversations through the wall. You will hear about the footsteps overhead.
That gap between what annoys people and what the building code regulates is worth understanding before you build, because one of those things is required and the other is a choice.
Two ways to comply
The code requires a dwelling unit to be separated from every other space in which noise may be generated. It gives two alternative paths to satisfy that, and a project uses one or the other.
- An ASTC rating of not less than 47, where the rating covers the separating assembly together with the adjoining constructions.
- An STC rating of not less than 50 for the separating assembly, with the adjoining constructions built to conform with the code's flanking article.
Both Part 3 and Part 9 of the code set identical numbers, so the standard does not shift with the size or type of the building. A fourplex and a larger residential building are held to the same separation between units.
Why there are two numbers
The difference between 47 and 50 is not a loosening. It is two different things being measured.
STC rates the separating assembly by itself, as a wall or floor tested in isolation. ASTC rates the whole path, including flanking sound, meaning noise that travels around the separating assembly through the adjoining walls, floors and ceilings.
So the ASTC number is lower because it is measuring more honestly. A party wall rated STC 50 in a laboratory can perform worse in a real building if sound moves through a continuous floor structure beside it. The STC path deals with that by prescribing how the adjoining constructions must be built. The ASTC path deals with it by measuring or calculating the actual result.
A wall does not perform in isolation. Sound will use whatever path you leave open, and the floor is usually the one that gets forgotten.
Where the code goes higher
Construction separating a dwelling unit from an elevator shaft or a refuse chute requires an STC of not less than 55, and the code offers no ASTC alternative for that case.
That makes sense on the character of the noise. A neighbour is intermittent and roughly predictable. A chute or a shaft produces sudden, repeated noise at any hour, from a source the resident has no relationship with and no ability to influence. The higher number reflects a different problem.
Secondary suites are held to less
This one matters for anyone converting or comparing project types.
In Part 9, a secondary suite may comply through a prescriptive assembly the code describes, involving sound absorbing material in the joist and stud spaces, resilient channel, and a minimum gypsum board thickness. Alternatively it may comply through an STC of not less than 43 or an ASTC of not less than 40.
Those are lower than the 50 and 47 required between separate dwelling units. The practical implication is that a house with a suite and a genuine multiplex are not built to the same acoustic standard, even where the unit count looks similar. A landowner moving from a suite arrangement to separate units should expect the separation requirements to step up, which is one of several differences covered in what is a multiplex.
The exclusion nobody mentions
Here is the part that explains the gap between code compliance and resident satisfaction.
The code's own appendix note states that the section has no requirements for the control of impact noise transmission.
Airborne sound, voices, music, a television, is regulated. Impact noise, footsteps, a dropped object, a chair scraping, furniture being moved, is not. And impact noise is overwhelmingly what generates complaints between stacked units.
This is a scope boundary rather than an oversight, and it is worth stating plainly rather than discovering later: a multiplex can fully comply with the code and still have a floor assembly that transmits every footstep from the unit above to the unit below.
What to do about it
If impact noise matters on your project, and on a strata multiplex where units are sold to separate owners it should, it has to be designed in.
The relevant decisions are structural. Floor assembly build-up, whether the ceiling below is decoupled from the structure above, what goes between the layers, and how the flooring is finished all bear on it. Every one of those is settled during design and executed during framing.
Retrofitting acoustic performance into a finished building means opening ceilings and reworking floors, which is disruptive, expensive, and generally less effective than doing it once at the right time. This is the same category of decision as the items in what drives construction cost on a small multiplex: spend that is invisible in the finished product and impossible to add cheaply later.
Code minimum as a decision
The numbers in the code are minimums. Treating them as a target is a choice, and on some projects it is the right one.
Where it deserves a harder look is on a strata multiplex. Units sold to separate owners who will live beside and above each other for years put the acoustic performance in front of people who did not design it and cannot change it. Complaints between owners in a small strata are a governance problem as much as a construction one, a dynamic covered in how a strata council works in a small multiplex.
A build-to-rent project is a different calculation, since the owner holds the asset and lives with the tenant turnover that poor separation produces. Either way, it is worth deciding deliberately rather than defaulting.
Questions for your designer
- Which compliance path are we using, ASTC or STC with prescriptive flanking, and why that one?
- What is the floor assembly between stacked units, and what does it do about impact noise given the code does not require anything?
- How is flanking handled where the separating wall meets the floor and the exterior wall?
- If this is a strata project, is code minimum the target, and has the owner made that decision knowingly?
Frequently asked
What sound rating does the BC Building Code require between units?
A dwelling unit must be separated from every other space in which noise may be generated by either an ASTC rating of not less than 47, or an STC rating of not less than 50 for the separating assembly with adjoining constructions conforming to the code's flanking article. Those are two alternative compliance paths, and Part 3 and Part 9 of the code set identical numbers.
What is the difference between STC and ASTC?
STC rates the separating assembly on its own. ASTC accounts for the whole path, including flanking sound that travels around the separating assembly through adjoining walls, floors and ceilings. The ASTC path uses a lower number because it measures more honestly. The STC path uses a higher number and controls flanking prescriptively through the code's flanking requirements instead.
Does the code cover footstep noise?
No. The code's own appendix note states that the section has no requirements for the control of impact noise transmission at all. Footsteps, dropped objects, and furniture being moved are impact noise, and they are the complaints we hear most from people living in new multi-unit buildings. Addressing them is a design decision made beyond what code compliance requires.
Is a higher rating required anywhere?
Yes. Construction separating a dwelling unit from an elevator shaft or a refuse chute requires an STC of not less than 55, and no ASTC alternative is offered for that case. Those are continuous noise sources of a different character from a neighbouring unit, which is why the standard is set higher.
Do secondary suites have a lower standard?
In Part 9, yes. A secondary suite may comply through a prescriptive assembly described in the code, or through an STC of not less than 43 or an ASTC of not less than 40. Those are lower than the 50 and 47 that apply between separate dwelling units, which is a meaningful difference if you are converting a suite arrangement into genuinely separate units.
What is flanking sound?
Sound that travels around a separating assembly rather than through it, using the adjoining walls, floors and ceilings as a path. A party wall built to a high rating can still perform poorly if sound moves through a continuous floor structure or a shared ceiling. This is why the code either measures the whole path through ASTC, or prescribes how the adjoining constructions must be built.
Does meeting code mean the units will be quiet?
It means they meet the minimum the code sets, which is a floor rather than a target. Because impact noise is outside the code entirely, and because the code's numbers are minimums, a building can fully comply and still generate complaints about footsteps from above. Going beyond the minimum is a design and budget decision, and it is easier to make before the assemblies are built.
When should acoustic decisions be made?
During design, because the assemblies that deliver sound separation between multiplex units are structural, not cosmetic. Adding mass, decoupling layers, or changing a floor assembly after framing means undoing finished work to get at the structure underneath. Retrofitting acoustic performance into a completed building is always more disruptive and less effective than designing it in from the start.
If you are planning a multiplex in Metro Vancouver and want the acoustic decisions made deliberately while they are still cheap to make, book a free consultation.
Written by
Founder & President
Mark Van Ek is the founder and president of Venture Pacific, bringing more than 40 years of Metro Vancouver and the Fraser Valley construction management experience and multiple Georgie and HAVAN awards to every custom home, renovation, and multiplex project.
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