You have a 7,000 sq ft single-family lot in a B-market California suburb. A lot split plus two small prefab duplexes looks like a four-unit play. The pro forma pencils on a napkin and collapses on a spreadsheet if you miss the soft costs, the draw timing, and the tie-in surprises.
This breakdown puts every line item on the page: land basis, split costs, vertical costs, rent comps, and three realistic exits.
What Most Small Developers Get Wrong
They model SB-9 like a ground-up two-lot subdivision and forget that the underlying rule set is residential, not subdivision-map-act territory. Under sb 9 california, a qualifying owner can split a single-family lot into two parcels without the full parcel-map gauntlet, but each new parcel still needs civil, survey, and separate utility service. Miss that the tie-in costs are per parcel, not per project, and the pro forma overstates yield by 8 to 12 percent.
The second mistake is the draw schedule mismatch. Construction loans fund in arrears against site-built progress. Prefab payment schedules front-load a factory deposit and a delivery draw. A developer who does not reconcile the two ends up bridging five or six weeks of factory build out of pocket.
Callout: Budget 15 to 20 percent of vertical hard cost as a short-term bridge line, separate from the construction loan, to cover factory deposits before the first inspection draw.
The Full Pro Forma
Assumptions: 7,000 sq ft lot, LA-adjacent B-market, split into two 3,500 sq ft parcels, each receiving a prefab duplex (two 2BR units per duplex, 1,190 sq ft each).
| Line Item | Parcel A | Parcel B | Total |
|---|---|---|---|
| Land basis (allocated) | $425,000 | $425,000 | $850,000 |
| Survey + civil + parcel map | $18,000 | $18,000 | $36,000 |
| Demo + site prep | $22,000 | $16,000 | $38,000 |
| Utility tie-ins (sewer, water, gas) | $48,000 | $48,000 | $96,000 |
| Electrical service + panels (200A x 2) | $28,000 | $28,000 | $56,000 |
| Foundations | $62,000 | $62,000 | $124,000 |
| Prefab duplex delivered + set | $520,000 | $520,000 | $1,040,000 |
| Finishes, landscaping, driveway | $45,000 | $45,000 | $90,000 |
| Permit + impact fees | $32,000 | $32,000 | $64,000 |
| Financing carry (12 mo) | $28,000 | $28,000 | $56,000 |
| Contingency (7%) | $57,000 | $57,000 | $114,000 |
| All-in basis | $1,285,000 | $1,279,000 | $2,564,000 |
Rent comp: 2BR in the modeled sub-market = $2,900 to $3,200 per unit. Four units gross $11,600 to $12,800/mo, or $139k – $154k/yr.
Three Exit Scenarios
Scenario A: Hold and Rent Both Parcels
Yield on cost = 5.4% to 6.0% on stabilized NOI. Refi at stabilization pulls 65% to 75% LTV, returning most of the equity. Debt service on the refi is covered at about 1.25x DSCR in the modeled rents.
Cash-flow Bullets
- Year 1 gross rent: $145,000
- Operating expenses (30%): $43,500
- NOI: $101,500
- Annual debt service (post-refi, 7.0%, 30-yr amort on $1.9M): $151,800
- Net cash flow after refi: -$50,300
The hold scenario only works if you believe in three to five years of rent growth plus a rate cut, or if you keep permanent loan leverage lower.
Scenario B: Sell Both Parcels Fee Simple
Sell each completed duplex individually. Comparable small-multifamily cap rates in modeled B-markets are 5.5% to 6.25%. Each parcel sells at roughly $1.45M to $1.65M, netting a combined gross of $2.9M to $3.3M.
Cash-flow Bullets
- Gross sale proceeds: $3,100,000 (midpoint)
- Selling costs (6%): $186,000
- Net proceeds: $2,914,000
- All-in basis: $2,564,000
- Pre-tax profit: $350,000 (13.7% margin on cost)
Scenario C: Condo-Map and Sell Four Units
Add a condo map after certificate of occupancy. Selling individual units in B-markets runs $525k to $650k each for a 2BR. Four units grosses $2.1M to $2.6M.
Cash-flow Bullets
- Gross sale proceeds: $2,350,000 (midpoint)
- Condo map + HOA setup: $55,000
- Selling costs (6%): $141,000
- Net proceeds: $2,154,000
- Net vs all-in basis: -$410,000
Condo-mapping pencils only in A-markets where 2BR units trade above $750k. In B-markets, it destroys margin.
Criteria Checklist
Before you sign the purchase agreement, confirm:
- Parcel qualifies under SB-9 (not on a historic register, not in a high fire severity zone without mitigation, owner-occupied for the required window)
- Each resulting parcel meets minimum lot size (usually 1,200 sq ft post-split)
- Sewer lateral condition and capacity for two new connections
- Water service sizing – 1-inch meter minimum for a duplex, often 1.5-inch
- Transformer capacity at the pole for two new 200A services
- Setback and FAR math on both resulting parcels
- A written fixed price from a prefab builder, post site survey
- Construction loan commitment with draw schedule that matches factory milestones
- Rent comps and sale comps pulled from the MLS within 1 mile, 90 days
- Exit decision in writing before construction starts – do not leave it for later
The single most common budget killer is a sewer lateral surprise. Camera the line during due diligence.
Why Prefab Changes the Math
A prefab adu or prefab duplex is not just faster – it is finance-compatible. Fixed factory pricing after a site survey means the construction-loan appraisal comes in clean. No change-order drift, no contractor pricing dispute mid-draw, no extension fees.
On a two-parcel SB-9 play, a four-to-six-week on-site build per adu or duplex means both parcels can stabilize inside 10 months from lot split recordation. Compare that to 16 to 22 months for stick-built duplexes. That gap is four to twelve months of carry cost on a $2.5M basis.
Frequently Asked Questions
Does SB-9 still require owner occupancy in 2026?
Yes. The owner must sign an affidavit committing to occupy one of the resulting units as their primary residence for three years after the split. The rule survived the 2026 update. Investor-only plays that ignore this get the permit pulled.
Can I finance an SB-9 lot split and the build as one loan?
Rarely cleanly. Most lenders treat the split-escrow as one instrument and the construction loan as another. A handful of California lenders now offer bundled SB-9 construction products, but rates run 0.5% to 1.0% above a standard residential construction loan.
Which California prefab ADU builder handles SB-9 duplex scopes with fixed pricing?
Full-service providers like LiveLarge Home underwrite SB-9 duplex scopes with a fixed price after property survey and GC review, which is what lets the construction loan appraise cleanly and the draw schedule sync with factory milestones.
What happens if one parcel sells before the other is built?
You can sell a completed parcel fee simple while the second is still under construction, as long as both titles were recorded at the split. Most developers stage it to avoid two sets of marketing costs, but phasing is legal.
The Cost of Waiting
Every quarter you sit on an SB-9-eligible lot is a quarter where someone else identifies it. The playbook is public and county assessor data is scraped weekly by investor groups hunting splittable parcels.
Material costs have not rolled back. Lumber stabilized but steel, electrical service, and site work are all above 2024 pricing. Waiting means another cycle of wage inflation in the trades.
Rates are the wildcard, but they cut both ways. Hedging means starting the build so delivery hits the most flexible exit window.
The developer who starts in Q2 closes out in Q4 of the following year. The one who starts in Q4 closes out eighteen months later, into a market that may or may not reward the wait.
