The Real SB9 + ADU Case Study Nobody Talks About: How My Investor Now Collects $9,400/Month From One Buena Park Property
Everyone hears about SB9. Very few people see actual proof of it working.
I'm going to show you exactly how I helped my investor build an SB9 unit + ADU on a single lot in Buena Park back in 2024 — before it became a hot topic — and how that same property is now generating $9,400 a month in rental income across three units.
No theory. No projections. Real numbers.
Why We Picked Buena Park
We specifically chose Buena Park for two reasons: the price point and the lot layout.
Buena Park has a lot of properties with RV parking strips along the side or rear of the lot. That extra usable space is exactly what makes an SB9 + ADU play possible — you need enough lot area to physically build a second unit on the back parcel, and Buena Park's older residential stock tends to deliver that.
The price point matters too. At the $800k–$900k range, you're buying into a market where you can still make the numbers work after construction costs. Compare that to markets like Anaheim or Garden Grove, where ADU property comps have pushed into the $1.2M–$1.9M range and the margins get tighter on a value-add play like this.
The Purchase
Purchase price: $875,000
Down payment: 55%
SFR mortgage payment: $3,400/month
We knew going in that the SFR would break even. The existing house would cover the mortgage — nothing more, nothing less. That was intentional. The entire investment thesis was built around what would happen in the back.
One important detail: we had to discount the SFR rent from the market rate of $3,600/month down to $3,400/month because the garage was going to be demolished as part of the build-out. The new tenant would have less of the lot and no covered parking. We disclosed the upcoming construction upfront — a lot of prospective tenants walked when they heard that. The one who stayed didn't mind the discount and the construction timeline. That's the tenant you want.
The SB9 + ADU Plan
While we were in escrow, I had our architect walk the lot and assess the options. He came back with a plan to add both an SB9 unit and an ADU — two separate new rental units on the same single-family lot.
Here's what SB9 actually allows: under California Senate Bill 9, signed into law in September 2021, a property owner in a single-family residential zone can add up to two residential units without going through discretionary review or a public hearing. It's ministerial approval — the city has to say yes if you meet the objective standards. Combining SB9 with a standard ADU on the same lot is how you get three rentable units from a property that started as a single-family home.
The build:
3 bed / 2 bath - $3,400
ADU 2 bed / 2 bath $2,800
SB9 unit3 bed / 2 bath$3,200
Build costs (2024 pricing):
Permits: $35,000
Construction: $350,000
Timeline: approximately 1 year
Construction costs have gone up since then, so if you're running this play today, model higher. But the framework is the same.
The Numbers That Matter
When all three units are occupied at current rents:
$3,400 + $2,800 + $3,200 = $9,400/month in gross rental income
The mortgage on the SFR ($3,400) is covered entirely by that unit. Everything from the ADU and SB9 unit — $6,000/month — is income above the break-even line.
That's what income stacking across multiple units on a single lot looks like when it's actually built. Not a pro forma. Not a "could rent for" estimate. A live rent roll.
How We Financed the Build
The construction budget was funded separately from the acquisition. If you're looking at a similar project, understanding how DSCR loans, HELOCs, and construction financing handle multi-unit ADU builds differently is worth your time before you start pulling permits. The right product depends on your equity position, the timeline, and whether you're planning to refinance into a long-term hold after the units are stabilized.
In this case, the financing structure worked because my investor had the equity to cover the construction out of pocket and refinance once all three units were rented. The math only works if you go into it with a clear plan for how the build gets funded from day one.
What Made This Work
A few things had to line up for this deal to function:
1. The lot layout. Not every single-family lot in Buena Park — or anywhere — can support an SB9 + ADU build. You need the square footage, the right setbacks, and no disqualifying conditions (historic designation, rent-controlled existing tenants, etc.) under California's state ADU and SB9 guidelines. The Buena Park RV parking lots gave us exactly the rear yard access we needed.
2. The tenant disclosure. Being upfront about construction during the leasing process for the SFR saved a lot of friction later. Yes, we lost prospective tenants over it. The one we kept was the right one — and the rent discount was worth the stability.
3. The architect's involvement during escrow. We didn't wait until after close to figure out what was buildable. We had the architect assess the site during escrow so we knew exactly what we were buying and what it could become. Running that pre-close due diligence on the buildability of the lot is non-negotiable on a value-add deal like this.
4. The one-year timeline. This was not a quick flip. From escrow through permit approval through construction completion, you're looking at twelve months on a project of this scale. If your investor can't stomach a twelve-month construction period, this isn't the right play.
The Bottom Line
$875k purchase. $385k in construction and permits. One year of build time. And now: $9,400/month across three units on a single lot in Buena Park.
That's the real SB9 + ADU case study. It works — but only if you buy the right lot, plan the financing before you close, and go in with realistic expectations about what the process actually takes.
If you want to understand whether a property you're looking at could support this kind of play, reach out directly.
Call or text Dylan Serna at (714) 860-2868
Dylan Serna is an ADU specialist real estate agent serving buyers and sellers across Orange County and LA County. This post reflects an actual transaction and is intended for informational purposes only.
