Fourplex
builder.
A fourplex crosses the line from single-family construction into multifamily. Type V residential, full CBC fire-rated assemblies, accessibility provisions under CBC Chapter 11A, EV-charging station readiness, and a meaningfully longer plan check all apply. The work is conventional wood-frame on a slab in most cases, but the regulatory layer is denser than a duplex — and the financing track is different from anything 1–3 units.
CA Lic. #1145233
“Fourplex is the smallest building where you are unambiguously running a multifamily project. Price it as one.”
What we mean by fourplex builder.
F-01 · Single-story garden fourplex
- Size
- 1,000 – 1,400 sqft per unit
- All-in
- $400 – $650 / sqft
- Schedule
- 14 – 18 months build
Single-story spread across the parcel. Simplest framing and MEP; usually the lowest cost-per-foot configuration.
F-02 · Two-story stacked fourplex
- Size
- 1,200 – 1,600 sqft per unit
- All-in
- $420 – $700 / sqft
- Schedule
- 14 – 20 months build
Two units down, two units up. Floor-ceiling acoustic and fire-separation assemblies become critical.
F-03 · Three-story townhome-style fourplex
- Size
- 1,400 – 1,800 sqft per unit
- All-in
- $450 – $750 / sqft
- Schedule
- 16 – 22 months build
Each unit is a vertical townhome with private entry. Party-wall detailing replaces floor-ceiling assemblies.
F-04 · Density-bonus fourplex (with affordable set-aside)
- Size
- Per program
- All-in
- $420 – $700 / sqft
- Schedule
- 18 – 26 months build
Use Density Bonus Law (Gov. Code §65915) to push to four units on R3 parcels that would otherwise allow three, in exchange for affordable set-asides.
Real California cost ranges.
All-in 2026 fourplex costs in California run $400–$750/sqft on flat lots, with Bay Area at the top of the band. Per-unit utility splits, EVCS provisioning, fire separation, accessibility, and longer plan check push soft costs to 16–22% of hard-cost subtotal — higher than duplex.
Utility splits (4 units)
$80k–$300k total for 4 units in sewer, water, gas, electric service splits. Service capacity sometimes requires a utility transformer upgrade ($15k–$60k).
Fire-rated assemblies
CBC Chapter 4 fire-rated separation walls, floors, ceilings $25k–$80k total. NFPA 13D / 13R sprinklers (geometry-dependent) $40k–$100k.
EVCS + Title 24
CALGreen requires EVCS readiness for new multifamily — typically 10–25% of parking spaces, with conduit and panel capacity. $20k–$60k. Title 24 per-unit heat pumps + PV $40k–$100k per unit.
Accessibility (CBC Ch 11A)
All units must be Type B accessible; some configurations require Type A units. Door widths, fixtures, switch heights. $5k–$15k per unit.
Foundation + framing
Type V wood frame on slab is most common — $80–$160/sqft for the slab + framing + sheathing package on flat lot.
Soft costs
16–22% of hard cost. Plan check + civil + structural are meaningfully heavier than duplex.
The permit path.
Fourplex projects are permitted under California Building Code (Group R-2) by the local building department. Plan check is longer than duplex (typically 16–32 weeks with corrections) because accessibility, fire-rated assemblies, sprinklers, and EVCS all attract separate review. Fire department review and utility provider sign-offs are mandatory.
- Building permit (architectural + structural)
- MEP permits
- Fire sprinkler permit (NFPA 13D or 13R per configuration)
- Title 24 energy compliance per unit
- CALGreen worksheet (4+ unit Tier 1 measures apply)
- Accessibility plan review (CBC Chapter 11A)
- EVCS provisioning per CALGreen 4.106.4
- Per-unit utility service permits
- Density bonus determination (if applicable)
How California code shapes the work.
California fourplexes are designed against CBC (R-2 occupancy), Title 24, CALGreen, and locally-adopted amendments. Fire-rated assemblies, residential sprinklers, accessibility, and EVCS are all enforced at plan check. In LA, SF, Oakland, and most peninsula jurisdictions, the fire marshal's review can be the long pole.
What the schedule actually looks like.
- Step 013 – 6 weeks
Feasibility + pro forma
Zoning, density bonus eligibility, parking, utility, pro forma.
- Step 0210 – 16 weeks
Schematic + DD
Plan layout, fire / acoustic strategy, EVCS layout.
- Step 038 – 14 weeks
Construction documents
Full permit set + fire + EVCS.
- Step 0416 – 32 weeks
Plan check + permit
Building + fire + utility tracks in parallel.
- Step 056 – 12 weeks
Sitework + utilities
Grading, 4-unit utility splits, slab prep.
- Step 0612 – 18 weeks
Framing → dry-in
Including fire-rated assemblies + sprinklers.
- Step 0718 – 28 weeks
MEP + finishes
Rough, insulation, drywall, finishes.
- Step 084 – 8 weeks
Commissioning + CofO
Per-unit + building-wide final inspections.
How we run this work.
Every fourplex starts with a written pro forma alongside the construction feasibility memo — rent / sale assumptions, hard cost band, soft cost band, financing scenarios. A fourplex is an investment-grade asset; the design has to make economic sense or we walk before pricing the build.
Fire separation, accessibility, and EVCS strategy are locked at design development. Owners see the per-unit cost impact of each before CDs lock so there are no surprises at plan check.
Construction runs on a design-build contract with named subcontractors and per-unit utility splits. The building is treated as one structure for the contract but each unit gets its own final inspection and CofO.
Frequently asked.
- What is the minimum lot size for a fourplex in California?
- Varies by jurisdiction. State law (SB 9, SB 423, Density Bonus) can push four units onto smaller lots than the underlying zoning allows. A 5,000-7,500 sqft R3 lot typically supports four units conventionally; SB 9 + ADU stacking can fit four units on R1 lots as small as 5,000 sqft.
- Do I need residential sprinklers?
- Almost always yes. NFPA 13D or 13R sprinklers are required for most Group R-2 fourplex configurations under CBC. The local fire marshal makes the final determination; we confirm at pre-application.
- How does Density Bonus Law affect a fourplex project?
- California Density Bonus Law (Gov. Code §65915) lets you build extra units on top of base zoning in exchange for affordable set-asides. On an R3 parcel zoned for three units, density bonus can typically push to four with one affordable unit. The bonus comes with waivers from parking and FAR limits when needed for feasibility.
- What financing is available for fourplex projects?
- Owner-occupant fourplex (you live in one unit) usually qualifies for residential construction loans under Fannie / Freddie conforming guidelines — much better terms than commercial. Pure investor 4-unit projects are also residential under most lenders' definitions, though terms tighten. 5+ units crosses into commercial.
- Can I keep the existing house and add three units?
- Sometimes — depends on local zoning, parking, and setback configurations. On many R1 parcels, the stacking is two SB 9 units + ADU + JADU, which can functionally produce four units without demolishing the primary residence. We model the option in the feasibility memo.
- What about parking?
- Parking requirements vary sharply by jurisdiction and have been relaxed by state law in many cases. Within ½ mile of major transit, AB 2097 eliminates minimum parking requirements entirely. Density bonus projects can request parking waivers. We confirm the actual requirement before pricing.
- How long does it take from contract to occupancy?
- Pre-construction + plan check + construction = 24–32 months total for a typical flat-lot four-unit project in a moderate jurisdiction. Bay Area dense cities and complex parcels reach 30–40 months.
Start with a feasibility memo.
Tell us about the parcel and the program. We'll come back with a written feasibility memo before any design work starts.
Want a real number for your Fourplex Builder job?
- We open the books on similar jobs from the last 24 months
- Hand-built estimate, not a software auto-quote
- Includes permits, finishes, and the boring stuff
3-day turnaround · Free