SSMUH Multiplex Structural Engineering in BC: What You Need

A four- or six-unit multiplex on a former single-family lot is not a larger house. It is a small multi-unit building: party walls doing fire, acoustic and structural duty at once, load paths that must stack through every storey, more lateral demand from a taller narrower form, and a foundation built tight to the property line. Bill 44 made these projects legal in most BC neighbourhoods; Bill 25 is standardising how municipalities implement them, with compliant zoning bylaws due by 30 June 2026.

Bill 44 and Bill 25 decide how many units your lot must be allowed to hold. The BC Building Code 2024 decides how they must be built. Provincial minimums run from three or four units on a typical lot to up to six on larger lots within 400 metres of a frequent-transit stop; confirm the number for your parcel with your municipality. Part 9 covers buildings of three storeys or fewer in building height with a building area not exceeding 600 square metres; past either limit the project becomes a Part 3 building. Most of the structural budget goes into party walls, transfers where units do not stack, soft storeys at parking, and foundations built beside neighbours.

What Bill 44 and Bill 25 mean for SSMUH multiplex structural engineering

Bill 44, the Housing Statutes (Residential Development) Amendment Act, received royal assent on 7 December 2023 and gave local governments until 30 June 2024 to update their bylaws. Inside a restricted zone, the provincial small-scale multi-unit housing requirements set a minimum of three to four residential units on lots previously zoned for a single detached house or a duplex, with the smallest parcels at the lower end.

Up to six units are required on larger parcels lying wholly or partly within 400 metres of a prescribed frequent-transit stop. The parcel-size thresholds and the definition of a qualifying stop are set provincially and applied through your local bylaw, so confirm the unit minimum for your specific lot before designing to it.

Bill 25, the Housing and Municipal Affairs Statutes Amendment Act, passed on 27 November 2025. It widened the definition of a restricted zone so that a zone containing any parcel limited to a single detached house or duplex must meet the minimum densities on all its lots. Local governments must adopt a compliant zoning bylaw by 30 June 2026 unless an extension is granted.

Some land is exempt, including large parcels, heritage-protected land, parcels not connected to both municipal water and sewer, land inside a designated transit-oriented area, and land where a qualified professional certifies that added density would significantly increase a hazard that cannot practically be mitigated. On the North Shore and in the Sea-to-Sky corridor that last exemption is not theoretical.

Neither bill changes the building code. They change what you may build, not what the structure must resist. And as bylaws are rewritten to conform with Bill 25, setbacks, height limits, site coverage and parking are all in play — those numbers set the building envelope, and the envelope sets the structure.

Part 9 or Part 3: the threshold that shapes everything

Part 9 of the BC Building Code 2024 applies to buildings of three storeys or fewer in building height, having a building area not exceeding 600 square metres, and used for residential and certain other listed occupancies. Exceed either limit and the building falls under Part 3, where design is fully engineered rather than prescriptive.

Both terms are defined, and the definitions decide projects outright. Building height in storeys is counted from the floor of the first storey, and the first storey is the uppermost storey whose floor sits not more than 2 metres above grade — grade being the lowest of the average finished ground levels along each exterior wall. On a sloping site, common through North Vancouver, West Vancouver and Coquitlam, that can turn what the drawings call a basement into a fourth storey.

Building area is the greatest horizontal area above grade, measured to the outside surface of the exterior walls or to the centre line of a firewall. Since a firewall divides a building into separate buildings for code purposes, a rowhouse-form multiplex is measured differently from six units under one roof.

Letters of assurance on a building this small

Letters of assurance apply to every Part 3 building. They are generally not intended for Part 9 buildings, although authorities having jurisdiction often elect to use them, and they are required for a Part 9 building with a common egress system that needs firewalls, and for individual components falling outside Part 9. A multiplex with numerous engineered components often ends up with a schedule covering the integrated structural design and field review for the whole building. Where an engineer takes on specific components only, the Schedule B should say so. Our guide to letters of assurance and Schedules B and C in BC sets out who signs each of the four schedules and when.

Party walls and honest load paths

Between dwelling units you need a fire separation with the required fire-resistance rating and an acoustic separation meeting the code's sound transmission requirements. The usual answers — double-stud and staggered-stud walls, discontinuous framing, resilient channel — all work by breaking continuity. Structure works the other way: shear walls want continuous sheathing, hold-downs want an uninterrupted path to the foundation, and drag elements want something to connect to. Decide early which leaf of each party wall is structural, put the shear sheathing, hold-down anchors and floor bearing on it, and keep the other isolated. Where the wall has to stay slender or non-combustible, cold-formed steel framing designed to CSA S136 is worth pricing.

Layouts are the second half of the problem. The ground unit wants open living space, the upper unit wants bedrooms, and walls stop lining up cleanly. Every misalignment becomes a transfer: a beam picking up the wall above and taking its load somewhere it can actually go. Transfers add depth, which eats ceiling height under a hard height limit; they concentrate load, which grows footings; and they multiply the connections needing field review. Engineered wood — LVL, PSL and glulam — usually carries them, but the least expensive multiplex is the one where bearing and shear walls stack, a lesson familiar from secondary suite and laneway house structural work.

Lateral demand: taller, narrower and full of openings

A multiplex is usually taller and narrower than the house it replaced, which increases overturning and drives more force into fewer wall segments, while the street elevation fills with windows, entries and a garage, leaving less wall to brace it.

Seismic demand has also moved. The National Building Code of Canada 2020 adopted Canada's sixth-generation seismic hazard model, and many buildings now see higher force and deformation demands than under the 2015 code; the detail is in the NBC 2020 seismic hazard changes. Metro Vancouver draws hazard from three sources at once — the Cascadia subduction zone, deep in-slab earthquakes beneath the Strait of Georgia, and shallow crustal faults — which is why seismic engineering here is a design exercise, not a table lookup.

Soft-storey conditions at parking and garage openings

The classic weakness is a ground storey far more open, and therefore softer and weaker, than the storeys above. Wide garage doors, tandem parking and a recessed entry can leave almost nothing to brace. Remedies include steel moment or portal frames designed to CSA S16, narrow high-aspect-ratio wood shear walls with heavy hold-downs designed to CSA O86, or a concrete ground storey designed to CSA A23.3. Each carries cost and coordination.

Sometimes the cheapest remedy is less parking. In designated transit-oriented areas a local government cannot require minimum off-street residential parking, which can take the garage out of the ground storey altogether. Outside those areas the treatment varies by municipality and is still being reworked as bylaws are updated, so confirm the requirement for your site before designing around it.

Foundations, shoring and neighbours on a tight lot

Six units on a lot that held one house push the building closer to the property lines, and the excavation follows. Anything deeper than the neighbouring footings raises shoring, underpinning, access agreements and monitoring — better resolved before you own the hole.

The code itself says when a geotechnical registered professional is needed on an otherwise Part 9 building: allowable bearing pressure below 75 kilopascals, foundations subject to slope instability, shoring, underpinning, deep foundations, foundation walls beyond the prescriptive limits, and retaining walls that support the building. A tight multiplex site hits several at once.

Soils vary sharply across the region. Fraser delta silts and sands under Richmond, Delta, parts of Surrey and South Vancouver amplify ground motion and can liquefy, often forcing deep foundations, while the glacial uplands of Burnaby, New Westminster and the North Shore benches are much stiffer. Get the geotechnical report before foundation design starts, because it can change the ground-floor strategy entirely.

SSMUH multiplex structural engineering across Metro Vancouver

Requirements are not uniform. Vancouver builds to its own Vancouver Building By-law 2025 rather than the provincial code, with its own transition arrangements — set out in our post on the Vancouver Building By-law 2025 changes — and that by-law lists additional circumstances in which registered professionals are required, including on Part 9 buildings. Its low-density residential zoning permits multiplexes, with the unit count depending on the zone and on whether units are secured as rental, so confirm the current numbers with the City.

Elsewhere the differences are just as real. Municipalities are still amending height, size, siting and parking rules in their small-scale multi-unit zones, and many apply development permit requirements on steep slopes, in floodplains and across natural-hazard areas that cover a large share of their single-family lots. Ask your authority having jurisdiction early.

What to have ready before the first engineering call

• A current legal survey showing grades, property lines and neighbouring buildings.
• Zoning confirmation, including the required unit count and whether the site is within 400 metres of a prescribed bus stop or in a transit-oriented area.
• An architectural concept, even a sketch, showing unit count, storeys and parking.
• Any geotechnical information already available for the site or the block.
• Your target permit submission date, so the structural engineering scope can be staged around it.

Working with MAF Engineering

MAF Engineering Ltd. is a consulting structural, seismic and civil engineering firm serving Greater Vancouver, the Fraser Valley, Sea-to-Sky and Vancouver Island. Principal Mir Afshar Niakouei, P.Eng. has more than 25 years of experience and is registered in British Columbia, Alberta and Ontario. We work on multi-family and custom residential projects across Metro Vancouver, and the earlier we see a multiplex massing, the more can be done about transfers, soft storeys and party walls. Call 778.840.9867 or get in touch to discuss scope and fee for your site.