How Much Does an ADU Cost in San Diego? Real Numbers From Real Builds

Ask three San Diego builders what an ADU costs and you will get three different answers — usually a wide range with no explanation of what sits behind it. Here is ours, with the actual math. We build ADUs across San Diego County every week, and the numbers below are the starting rates we quote from, the worked examples we run with homeowners at our Mira Mesa office, and the rent math our clients actually use to decide.

Our Starting Rates — The Number We Quote From

For a new detached ADU, XLNC starts at $300 per square foot for units 600 square feet and up, and $350 per square foot under 600 square feet. Smaller units cost more per foot because the kitchen, bathroom, and utility hookups cost roughly the same whether the unit is 400 square feet or 1,200. That is the starting construction rate — site work, permits, and design sit on top of it, and we break those out line by line before anyone signs.

For the other routes, our planning ranges are the ones on our ADU cost guide: garage conversions typically $80,000 to $150,000, junior ADUs $40,000 to $80,000. A garage conversion costs less because the slab, walls, and roof already exist — the money goes into leveling, insulation, plumbing, and making it a legal, comfortable home.

Worked Examples: What Real Sizes Actually Cost to Start

Take the starting rate and run it against the sizes we build most:

Unit	Size	Starting construction cost
Studio	450 sq ft	$157,500 (at $350/sq ft)
One-bedroom	600 sq ft	$180,000 (at $300/sq ft)
Two-bedroom	800 sq ft	$240,000 (at $300/sq ft)
Two-bedroom, large	1,200 sq ft	$360,000 (at $300/sq ft)

Read those as starting points, not quotes. The final number for your lot depends on three things, and we price all three in the estimate — never as a surprise later.

The Three Numbers That Decide Your Quote

1. Size times rate. The table above. A simple rectangular footprint with a standard roofline costs less than a complicated shape with the same square footage.

2. Site work. This is where San Diego lots bite. A flat Mira Mesa lot with clear side access is the easy case — and it is the case on our current Mira Mesa (92126) detached ADU build, video walkthrough. A sloped lot, tight equipment access, mature trees, or poor soil means grading, retaining, and foundation work that can run into the tens of thousands on otherwise identical units.

3. Utilities. Running water, sewer, power, and gas from your house to a detached unit is a real line item, and distance is the driver. A unit 20 feet from the house is a different job than one 80 feet away across a sloped yard. If your electrical panel needs an upgrade, that goes in the estimate too — not in a change order halfway through.

What the Rate Does Not Include (Budget for These Too)

• Design and engineering — architectural plans, structural calculations, energy compliance documents, sometimes a soils report.
• Permit and plan-check fees — charged by the city.
• Impact fees — but note this: California law exempts ADUs under 750 square feet from impact fees, which can save thousands. That is one reason the 600–749 sq ft one-bedroom is our most-built size.
• A contingency of 10 to 15 percent. Every honest builder will tell you to hold this. Old lots hide old problems — a cracked sewer lateral, unmapped easements — and the contingency is what keeps a surprise from becoming a crisis.

On timing: plan on 2 to 3 months for permits and 5 to 6 months to build once permits are in hand. Time affects cost directly — if you are paying rent elsewhere or carrying a construction loan, every month of delay is money. That is why we run design, permitting, and construction as one team instead of handing you off between three companies.

The Rent Math: Why the Number Works

A well-built San Diego ADU can rent for up to $3,500 a month depending on size and neighborhood. That is up to $42,000 a year in gross rent. Against a $180,000 starting construction cost for a 600 sq ft one-bedroom, that is a payback measured in years, not decades — and the rent keeps coming long after the build is paid off.

And rent is only one reason people build. The same unit houses aging parents, adult children, or a home office — and adds usable square footage that stays with the property. For the full breakdown of types, see our complete ADU cost guide, and if you are weighing a garage instead of a new build, our garage conversion guide covers that route.

Build Your ADU With XLNC Builder

XLNC Builder is a licensed, bonded, and insured San Diego contractor (CSLB Lic. #1153455) specializing in custom ADU construction. Design, permits, and construction — one team, milestone payments, no big upfront surprises.

Want a real number for your lot? Bring a backyard photo and your lot address to our office at 9400 Activity Rd, Suite G, San Diego — our free Backyard Report check will tell you what fits, what it starts at, and what your lot allows. Or call (619) 836-0099 and request a free quote.

Regards, INIT XLNC Builder 9400 Activity Rd, Suite G, San Diego, CA 92126 +1 (619) 836-0099

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Frequently Asked Questions

What is the average cost to build an ADU in San Diego?

Most San Diego ADUs land between $80,000 and $300,000+. Garage conversions and junior ADUs sit at the lower end; detached units run $150,000 to $300,000 or more. XLNC's starting rate for a new detached ADU is $300 per square foot for units 600 square feet and up — $350 per square foot under 600.

What is the cheapest way to add an ADU?

A junior ADU inside your existing home is the lowest-cost route — typically $40,000 to $80,000 — followed by a garage conversion at $80,000 to $150,000. Both reuse structure you already own.

Do ADUs under 750 square feet really skip impact fees?

Yes — California law exempts ADUs under 750 sq ft from impact fees. Permit and plan-check fees still apply. Confirm the current rules with your builder, since fee schedules change.

How long does it take to build an ADU in San Diego?

Expect 2 to 3 months for permits and 5 to 6 months of construction once permits are issued — roughly 8 to 9 months from a signed design to move-in when the process runs cleanly.