Building Types | Duplex
Duplex: Two Units, One Lot, Lots of Permutations
The duplex sits at the bottom of the missing middle scale. It is also the form built most often when an owner converts a single-family lot under Bill 44 — because it is the simplest, the cheapest per unit, and the easiest to finance.
Key Takeaways
- ✓Four configurations: side-by-side, front-to-back, stacked, duplex-plus-suite.
- ✓Most BC SSMUH bylaws permit a duplex anywhere a single-family home was previously allowed.
- ✓Half-duplex strata is the dominant tenure for new BC duplexes.
- ✓Per-unit construction cost is usually lower than a triplex or fourplex on the same lot.
The Four Configurations
Side-by-side duplex
Two units share a party wall along the long axis of the lot. Each unit gets its own front entry, full-height building, and direct access to a back yard. Most common on wide lots (40 ft+).
Front-to-back duplex
One unit faces the street, the other faces the lane. A side hallway or breezeway separates them. Works on narrow lots without sacrificing private outdoor space.
Stacked duplex (up/down)
One unit on the lower level, one above. Typically the simpler and cheaper build but produces inferior unit quality — the lower unit lives darker and the upper has no ground floor.
Duplex + secondary suite
A duplex with one or two basement suites. Counted as up to four dwelling units for tax and zoning, but built as a duplex shell. The most common pre-Bill 44 typology in Vancouver.
Code Triggers Specific to Duplex
BC Building Code Part 9 governs duplexes. The party wall between units must achieve a one-hour fire-resistance rating and meet Sound Transmission Class 50 (or be demonstrated equivalent under STC field testing). Each unit needs a separate primary egress route. Each unit gets its own service connections — water, sewer, electrical — unless the strata plan explicitly shares them.
Compared to triplex and fourplex, duplex avoids the multi-unit corridor rules that trigger Part 3 of the code. That is what keeps duplex construction cost lower per unit.
Strata or Rental: The Tenure Question
Half-duplex strata is the standard exit for most BC builders. Two strata lots, no common property beyond the party wall and roof, minimal strata corporation overhead. The Strata Property Act process for a two-lot residential strata is the simplest version of stratification available in BC.
Holding the duplex as a single-title rental is also common, especially when the owner wants to retain one unit for a family member. See rental vs strata for the decision tree.
When Duplex Is the Wrong Answer
On a 4,000 sq ft Vancouver lot at 0.7 FSR, a duplex produces two units of about 1,400 sq ft each — large units, full lot utilisation, two market-rate sales. On the same lot, a fourplex produces four units of about 700 sq ft. The fourplex doubles the unit count without any change in zoning. For owners optimising for unit count, the duplex is leaving units on the table.
For owners optimising for simpler construction, lower stratification overhead, or larger family-sized units, the duplex is the right call. See our fourplex page for the comparison.
Best For
- ✓ Owners who want to keep one unit and sell or rent the other.
- ✓ Lots over 50 ft wide where side-by-side configurations work easily.
- ✓ Builders prioritising family-sized units (1,200–1,600 sq ft) over unit count.
Usually Fails When
- ✕ You have a small lot below 4,000 sq ft and need to maximise unit count for the pro forma to work.
- ✕ You expect duplex strata to escape ongoing strata corporation obligations entirely.
- ✕ You assume duplex avoids fire-rated party-wall construction — it does not.
What To Verify Before Spending Money
- → Your lot's SSMUH unit allowance — duplex is universally permitted but FSR sets the building size.
- → Whether your municipality requires a Development Permit even for compliant duplexes.
- → The half-duplex strata process with a BC notary or real estate lawyer.
Frequently Asked Questions
Is a duplex still missing middle?
Why are duplexes the most common Bill 44 outcome?
Can a duplex be stratified?
What lot size do I need for a duplex?
Why are duplexes "under-loved" for missing middle?
Official Sources Referenced
Screen Your Lot for Missing Middle
Enter any BC address to see what Bill 44 SSMUH unit count, lot coverage, and FSR your parcel actually qualifies for.