The Search: How to Find a Duplex + ADU Property in San Diego (in 30 Minutes a Day)

The Search: How to Find a Duplex + ADU Property in San Diego (in 30 Minutes a Day)

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 there's a catch: the whole strategy is worthless if you can't find the right property.

This is the part most people skip. They learn the financing, get excited, open Zillow, scroll through pretty single-family homes, and eventually give up. So let me give you the exact search filter I use to find duplexes with ADU potential — the same one I run for every first-time buyer who sits down with me. It takes 30 minutes a day, it's free, and it works.

Step 1: Filter for Multi-Family — Skip Single-Family Entirely

Open Zillow or Redfin and set the filter to Multi-Family (sometimes labeled "2+ Units" or "Duplex/Triplex/Fourplex"). That one click cuts 95% of the noise.

Here's why it matters: this whole series is built around FHA's owner-occupied multi-family financing, which only works on 2–4 unit properties. The moment you start scrolling single-family homes, you've left the strategy behind. Multi-family is a small fraction of San Diego County listings on any given day — and that scarcity is the opportunity, because fewer buyers are hunting the same properties.

Step 2: Lot Size — Minimum 5,000 Square Feet

In the filters, set a minimum lot size of 5,000 square feet. Most San Diego residential lots fall in the 5,000–6,000 range, and that's the practical floor where an ADU build actually pencils out. Smaller lots run into real constraints — setbacks (typically 4 feet from rear and side lines), coverage limits, privacy buffers, and sometimes required parking in overlay zones.

One nuance worth knowing: California law (SB 9 and AB 68) has eliminated minimum lot-size requirements for ADUs, so you can technically build on smaller lots. But "legally allowed" and "physically feasible" are two different things. On a 4,000 sq ft lot with an existing duplex, there's often not enough usable backyard left for a real ADU. Filter for 5,000+ and you'll save yourself a lot of disappointing property visits.

Step 3: Hunt for Ugly

This is the most important step — and the one new buyers get most wrong.

Scroll right past the renovated, pretty listings. The duplex with the fresh paint, the staged photos, the granite counters? That's the one everyone else is bidding on, and the seller already captured most of the upside before they listed it.

You want the opposite. Look for:

• Dated photos that look straight out of 2008
• Original kitchens and baths — linoleum floors, oak cabinets
• Overgrown landscaping and visible exterior wear
• No staging, no professional photography
• A motivated seller — "relocating," "estate sale," "as-is"

These properties scare off most buyers, and that's exactly what creates the opening. Ugly equals upside. You're not buying the finishes — you're buying the bones, the lot, and the ADU potential, with the renovation budget as your value-add play instead of the seller's. A dated $850,000 duplex with a big backyard beats a renovated $1.1M duplex with no room for an ADU every time. The math favors the ugly one.

Step 4: Check the Zoning — Look for RM Zones

Once a property clears Steps 1–3, confirm it's actually zoned for what you want to do. Copy the address into the City of San Diego zoning lookup or the SANDAG GIS portal and check the zoning code.

You want RM zones. Here's the quick translation:

• RM-1-1, RM-1-2, RM-1-3 — Residential Multiple Unit, lower density: your sweet spot for a duplex + ADU
• RM-2-5, RM-3-7, RM-3-9 — higher-density multi-family: great for triplex and fourplex plays

RM zones are your green light — they explicitly permit multiple dwelling units, and in 2026 they let you build up to two detached ADUs on a multifamily lot, plus convert non-livable interior space into additional units.

Yellow light: if it comes back RS (Residential Single-Family), it's not a deal-breaker, just a more limited play — you can usually still add one ADU and one Junior ADU, but no extra dwelling units beyond that.

Red flags: Coastal Overlay Zones, slope/hillside overlays, and historic districts. None of these outright block ADUs, but they add review time and design constraints. If you're new to this, I'd skip those properties until you've got a few deals under your belt.

How to Run the Search

Here's the daily rhythm:

1. Open Zillow or Redfin with your Multi-Family + 5,000 sq ft filter saved
2. Scroll new listings, zeroing in on the ugly ones
3. Pick 2–3 candidates that catch your eye
4. Look up zoning on the SANDAG portal for each address
5. Save the RM-zoned ones to a spreadsheet or saved-search list
6. Drive by in person within a few days — photos lie, and a quick drive-by tells you everything about the real neighborhood and lot

Thirty minutes a day, every day, for 60–90 days. That's the cadence that finds you a property 90% of buyers and agents walk right past.

What 90% of Buyers Get Wrong

Most people run this backwards. They look at single-family homes first "just in case," get distracted by the renovated listings, skip the zoning step entirely, and quit after a few weeks of not finding "the perfect one."

Flip all of that. You're not looking for a perfect home — you're looking for the right structural opportunity. Bones, lot size, and zoning are what matter. Finishes and aesthetics don't, yet — that's what your renovation budget is for.

One More Catch — Location

This strategy works in San Diego, but not equally well in every neighborhood. Some areas have the right zoning, rents, ADU permitting, and lot sizes all lined up. Others have all the wrong ones.

In Part 4, I'll break down the three San Diego neighborhoods where this Duplex + ADU play still pencils out in 2026 — where the numbers actually work, where ADU permits get approved quickly, and where I'd focus your daily searches.

In the meantime, if you'd rather not run this search alone — or you've found a property and want a second set of eyes on the bones, the lot, and the zoning before you make a move — reach out to the Routt Home Team here and we'll dig into it with you. And follow @askwill on Instagram to catch Part 4 when it drops.

Coming Up in the San Diego Real Estate Cheat Code Series

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.