Part 3 of the San Diego Real Estate Cheat Code Series. The exact 4-step search filter I use to find duplexes with ADU potential — Multi-Family + 5,000 sq ft + ugly + RM zoning.
Part 3 of the San Diego Real Estate Cheat Code Series
See it live on Instagram: @askwill
In Part 1, I showed you how to live nearly for free in San Diego with a duplex and about $30,000 down. In Part 2, we layered in the ADU Play — turning one property into three rental paychecks.
But here's the catch: the entire strategy is worthless if you can't find the right property.
This is the part most people skip. They learn the financing strategy, get excited, then open Zillow and scroll through pretty single-family homes until they give up. Stop doing that. Here's the exact search filter I use to find duplexes with ADU potential in San Diego — the same one I run for every first-time buyer who walks into my office.
It takes 30 minutes a day. It's free. And it works.
Open Zillow or Redfin. Set the filter to Multi-Family (sometimes labeled "2+ Units" or "Duplex/Triplex/Fourplex").
This one click eliminates 95% of the noise.
Single-family homes are a different game with different math. The Cheat Code Series is built around FHA's owner-occupied multi-family financing — which only works for properties with 2-4 units. The moment you start scrolling through single-family homes, you've already left the strategy behind.
In San Diego County, multi-family inventory is a small fraction of total listings on any given day. That scarcity is actually the opportunity — fewer competitors hunting the same properties.
In the search filters, set a minimum lot size of 5,000 square feet.
Why 5,000? Most San Diego residential lots fall in the 5,000–6,000 square foot range, and that's the practical minimum where an ADU build genuinely pencils out. Smaller lots create real constraints:
Worth knowing: California state law (SB 9 and AB 68) has eliminated minimum lot size requirements for ADUs — meaning you can technically build an ADU on smaller lots. But "legally allowed" and "physically feasible" are different things. On a 4,000 sq ft lot with an existing duplex, there's often not enough usable backyard left to put a real ADU. Stick with 5,000+ sq ft as your filter and you'll save yourself a lot of disappointed property visits.
This is the most important step — and the one most new buyers get wrong.
Scroll past the renovated, pretty listings. The duplex with the freshly painted exterior, the staged photos, the granite countertops — that's the property other buyers are bidding on. The seller already captured most of the upside before listing it.
You want the opposite. Look for:
These properties scare off most buyers — which is exactly what creates the opportunity. Ugly = upside. You're not buying the cosmetic finish; you're buying the bones, the lot, and the ADU potential. The renovation budget is your value-add play, not the seller's.
A duplex listed at $850,000 with dated finishes and a big backyard is a far better deal than a renovated $1.1M duplex with no room for an ADU. The math always favors the ugly one.
You've found a property that passes Steps 1-3. Now verify it's actually zoned for what you want to do.
Copy the property address. Open the City of San Diego's zoning lookup tool or the SANDAG GIS portal. Paste the address and look at the zoning code.
You want RM zones. Here's what they mean:
RM zones are your green light. They explicitly permit multiple dwelling units, and in 2026 they allow you to build up to two detached ADUs on a multifamily lot (plus convert non-livable interior space into additional units).
Yellow lights: If the property comes back as RS (Residential Single-Family), it's not necessarily a deal-breaker — but it's a more limited play. You can still build one ADU and one Junior ADU on most RS lots, but you can't add additional dwelling units.
Red flags to watch for: Coastal Overlay Zones, slope or hillside overlays, and historic districts. These zones don't prevent ADUs, but they add review time and design constraints. If you're new to this strategy, avoid those properties until you've done a few deals.
Here's the daily workflow:
30 minutes a day, every day, for 60-90 days. That's the rhythm that finds you a property 90% of buyers and agents completely miss.
Most buyers approach the search backwards. They:
The right approach is the opposite. You're not looking for a perfect home — you're looking for the right structural opportunity. Bones, lot size, and zoning matter. Finishes and aesthetics don't (yet — that's your renovation budget).
This entire strategy works in San Diego — but it doesn't work equally well in every neighborhood. Some areas have the right zoning, the right rents, the right ADU permits, and the right lot sizes. Others have all the wrong factors.
In Part 4, I'll reveal the three San Diego neighborhoods where this Duplex + ADU play still pencils out in 2026 — and where you should specifically focus your daily searches. These are the neighborhoods where the numbers actually work, where ADU permits get approved quickly, and where you'll find your deal.
Follow @askwill on Instagram to catch it when it drops, and check back on the Hello San Diego blog when Part 4 publishes.
Part 4: The 3 San Diego Neighborhoods Where the Duplex + ADU Play Still Pencils Out — Where to actually run this strategy, with rent comps, recent deals, and the specific ZIP codes worth your daily search.
Part 5: Coming soon.
This article is for informational purposes only and does not constitute financial, legal, or tax advice. ADU regulations, FHA limits, zoning rules, and rental income calculations vary by lender, property, and zone. Consult a licensed real estate professional, lender, and the City of San Diego planning department before making any home-buying or construction decisions.
Part 2 of the San Diego Real Estate Cheat Code. How to add an ADU to your duplex and turn one property into three paychecks.
Part 1 of the San Diego Real Estate Cheat Code series. The $190K down payment myth is keeping buyers on the sidelines — the real number is closer to $30K if you know the FHA duplex play.