Will Building an ADU Raise My Property Taxes in San Diego? (2026)
Yes — but only on the ADU itself, not your whole house. California does not reassess your existing home when you build an ADU. The county assesses just the new unit at today’s value and adds it to your bill. In San Diego that usually means roughly $1,200–$4,000 more per year, depending on the ADU’s size and type — far less than the rent most units bring in.
I’m Daniel Dechner at IL Total Design & Build. We design and build ADUs across San Diego, and after “what does it cost,” this is the question I hear most: “If I build an ADU, are my property taxes going to blow up?”
It’s a fair worry, because in San Diego the gap between what people pay in taxes and what their home is worth is enormous. A house bought in 1999 might still carry a Prop 13 tax basis under $400,000 while it’s worth $1.6M today. Homeowners hear “reassessment” and picture the county revaluing the entire property at market value — turning a $3,500 tax bill into $18,000 overnight.
That fear is understandable, and it’s wrong. Here’s exactly how the tax on a new ADU works in San Diego, what the increase actually looks like in real numbers, the surprise bill that catches people off guard, and a couple of myths worth killing.
Why your whole home is not reassessed
Two completely different things can change your property taxes: building something new, and transferring the property to someone else. People mix them up constantly. This article is about building. (For the inheritance and parent-to-child side, see our guide to Prop 19 & ADUs in San Diego.)
Under Proposition 13, your home’s assessed value is locked to its “base-year value” and can only rise about 2% a year — no matter how much the market climbs. When you build an ADU, California treats only the ADU as new construction. The county assesses the value of that new unit at today’s rate and adds it to your existing assessment. Your main house keeps its low Prop 13 basis, untouched. Assessors call the result a “blended” assessment: old house at its protected value, plus the new ADU at current value.
So the only thing that gets taxed at today’s value is the square footage you just added — not the home you’ve owned for 20 years.
How the county figures the ADU’s value
The Assessor adds the market value the ADU contributes as new construction. Two things surprise people here:
- It’s based on the structure’s value, not your total project cost. Your all-in budget includes soft costs, design, financing, site work, and a builder’s margin. The Assessor is valuing the finished unit’s contribution to the property — which is usually less than what you spent to build it. It is common for the assessed add to land below your total project cost.
- Your rent doesn’t change it. The Assessor values the construction, not your income. Renting the ADU for $2,500 a month does not raise your assessment. (Rental income is taxed separately as income — a CPA question, not a property-tax one.)
San Diego’s effective property-tax rate is roughly 1.1%–1.25% of assessed value in most areas — the 1% Prop 13 base plus local voter-approved bonds, which vary by tax-rate area. Apply that rate to the ADU’s added value and you have your annual increase.
Real San Diego numbers (2026)
| ADU type & size | Approx. assessed value added | Approx. added tax / year |
|---|---|---|
| Garage conversion, ~450–600 sq ft | ~$90,000–$150,000 | ~$1,000–$1,800 |
| Detached ADU, ~700–800 sq ft | ~$170,000–$250,000 | ~$1,900–$3,000 |
| Detached ADU, ~1,000–1,200 sq ft | ~$260,000–$370,000 | ~$2,900–$4,300 |
A garage conversion typically adds the least, because you’re improving an existing structure rather than building new square footage from the ground up. A full 1,200 sq ft detached unit adds the most. Either way, the increase is tied to the unit — not a reset of your whole tax bill.
A Clairemont homeowner builds a 750 sq ft detached ADU. Their main home was bought in 2001 and carries a Prop 13 basis around $350,000, with a tax bill near $4,000/year. The county assesses the new ADU at about $210,000 in added value. At a ~1.15% effective rate, that’s roughly $2,400 more per year — bringing the total to about $6,400. The $330,000 the home has gained in market value over those years? Still untaxed. And the ADU rents for around $2,500/month, so the added tax is covered in about five weeks of rent.
The supplemental bill nobody warns you about
Here’s the one that genuinely catches people off guard. When your ADU is finished, you don’t just see a higher number on next year’s regular tax bill. You also get a one-time supplemental tax bill, mailed off-cycle, covering the period from the date the unit was completed (or received final sign-off) through the end of the current fiscal year.
It’s prorated by the months remaining and tied to your tax-rate area, and it arrives separately from your normal bill — so it feels like a surprise even though it’s routine. After that, your regular secured bill simply reflects the new blended total. Budget for the supplemental so it doesn’t sting.
Three myths worth killing
- “Building an ADU reassesses my whole property.” No. Only the ADU is assessed at current value; Prop 13 protects the rest.
- “There’s an ADU property-tax credit or exemption in California.” There isn’t a special statewide ADU tax break — that’s a persistent myth. The benefit is structural: Prop 13 keeps the reassessment limited to the new unit. Don’t count on a “credit” that doesn’t exist.
- “Renting it out raises my assessment.” No. The Assessor values the construction, not your rent. (Rental income affects your income taxes, where you may also deduct expenses and depreciation — ask your CPA.)
Want your real number before you build?
Tell us your lot and goals. We’ll scope what’s buildable, what it costs, and roughly what to expect on the tax side — free, no obligation.
How we fit in
We don’t give tax advice, but we build the unit the whole calculation rests on — and the construction value is what the Assessor uses to figure the ADU portion. At feasibility we sort out the things that actually move your number:
- Whether a garage conversion (lower assessed add) or a detached unit makes more sense for your lot and goals.
- Realistic construction cost, since that drives both your budget and the assessed value. See our 2026 San Diego ADU cost guide.
- How to phase and pay for it. See ADU financing in San Diego.
- Your city’s specific rules, in our San Diego ADU regulations guide.
Quick reference
- Whole-home reassessment? No — only the ADU’s added value is assessed; your home keeps its Prop 13 basis.
- Typical increase: ~$1,200–$4,000/year, scaling with the unit’s size and type.
- Based on: the ADU’s construction/market value — not your home’s value and not your rent.
- Watch for: a one-time, prorated supplemental bill after completion.
- Myth check: no special California “ADU tax credit”; the real benefit is Prop 13 limiting the reassessment.
- Always: confirm your exact figure with the San Diego County Assessor.
