Published: 3 Oct 2026 · Last verified: 3 Oct 2026 · Next review: Jan 2027 Cost figures are re-checked against permit records quarterly. Changes are logged in Corrections.
Every other post on this site is anchored to Los Angeles County. This one is not, and the reason is worth more than the number.
San Diego County has something Los Angeles does not: a permit fee spread so wide that your city line matters more than your floor plan.
Inside the City of San Diego, building plan check and inspection on an 800 sq ft detached ADU run about $16,500, roughly $21 per square foot (IB-501). Twenty-five miles north in Encinitas, the city waives all of its Development Services fees for ADUs (Encinitas ADU guide). Same state law. Same building code. Same drawings. Five figures versus zero.
So this post does two things. First, the standard six-phase breakdown for an 800 sq ft detached ADU in the City of San Diego — $344,000, or $430 per square foot. Then the part nobody publishes: what that identical build costs in four different jurisdictions inside the same county.
The short answer
| 800 sq ft detached ADU · City of San Diego · 2026 | Amount |
|---|---|
| Total build cost | $344,000 |
| Cost per square foot | $430 |
| Largest single line | Systems + interior, $158,000 |
| Soft costs vs Los Angeles County | +$4,000, almost entirely permit fees |
| Same build in Encinitas | ≈ $326,600 |
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The full line-item breakdown
Six phases. The same six used on every build in this series.
| Phase | Cost | Share |
|---|---|---|
| Soft costs (design, permits, plan check, impact fees) | $36,000 | 10% |
| Site work + foundation | $40,000 | 12% |
| Framing + structure | $50,000 | 15% |
| Building envelope | $45,000 | 13% |
| Systems + interior | $158,000 | 46% |
| Landscaping & hardscape | $15,000 | 4% |
| Total | $344,000 | $430 / sq ft |
Notice the shape is identical to Los Angeles. Systems and interior is 46% here too. One kitchen, one bathroom, one heat pump, one electrical service — those do not care which county you are in.
What changed between LA and San Diego is the hard-cost rate and the permit bill, not the structure of the spend.
Where San Diego differs from Los Angeles
| Phase | LA County | City of San Diego | Difference |
|---|---|---|---|
| Soft costs | $32,000 | $36,000 | +$4,000 |
| Site work + foundation | $38,000 | $40,000 | +$2,000 |
| Framing + structure | $48,000 | $50,000 | +$2,000 |
| Building envelope | $42,000 | $45,000 | +$3,000 |
| Systems + interior | $147,000 | $158,000 | +$11,000 |
| Landscaping & hardscape | $13,000 | $15,000 | +$2,000 |
| Total | $320,000 | $344,000 | +$24,000 |
| Per sq ft | $400 | $430 | +$30 |
San Diego runs about 7% above the Los Angeles County median on the same unit. Our estimate is that most of that is subcontractor rates in a smaller, tighter trade market — not land, not materials, not regulation.
The regulation difference shows up somewhere else entirely.
The part nobody publishes: four cities, one build
Here is the same 800 sq ft detached ADU, same drawings, same finishes, priced in four San Diego County jurisdictions. Only the soft costs move.
| Jurisdiction | Permit fees ≈ | Design & engineering | Other soft (our model, unitemized) | Soft costs total | Build total |
|---|---|---|---|---|---|
| City of San Diego | $16,500 | $14,000 | $5,500 | $36,000 | $344,000 |
| County (unincorporated) | $4,400 | $14,000 | $5,200 | $23,600 | $331,600 |
| Carlsbad | $3,000 | $14,000 | $6,000 | $23,000 | $331,000 |
| Encinitas | $0 | $14,000 | $4,600 | $18,600 | $326,600 |
The “Other soft” column is the remainder of our modeled soft-cost total after permit fees and the $14,000 design line — impact, school, utility, and miscellaneous fees from our model. It is not itemized per jurisdiction because those fees are modeled, not taken from a published per-jurisdiction schedule.
A $17,400 swing on identical plans, decided entirely by which side of a street your lot sits on.
The City of San Diego number is not an error. The city bills a detached ADU over 500 sq ft at its single-dwelling rate: $8,085.26 plan check plus $8,401.90 inspection (IB-501, Aug 2026). The County charges $1,940 + $0.410/sq ft for plan review and $1,677 + $0.564/sq ft for the permit, about $4,400 at 800 sq ft (PDS-613, effective 10 Jul 2026). Carlsbad puts most ADU permit fees at about $2,000 to $4,000 (City of Carlsbad). Encinitas waives its Development Services fees (Encinitas ADU guide). On an 800 sq ft unit that is the difference between a roughly $16,500 permit bill and $0 in city permit fees. All fee figures checked 3 Oct 2026.
Escondido goes further
Escondido publishes four free pre-approved ADU plans (PAADU program). If one of them fits your lot, most of the $14,000 design and engineering line disappears before you start. Escondido also waives wastewater, water, traffic, public facility, drainage, park, and other development impact fees for ADUs built under Article 70 of its zoning code (2025 Fee Guide).
Stack the free plans against the City of San Diego’s permit load and, by our rough estimate (not a modeled line above), the soft-cost gap between the two jurisdictions could approach $27,000 — on a build where not one stick of lumber has changed.
The City of San Diego has its own answer
The city does not run its own plan library. Instead it accepts pre-approved plans from the County of San Diego, Chula Vista, and Encinitas programs, and applications that use them get a 30-day review (City of San Diego ADU page). You pay the plan provider’s licensing cost. For scale, Carlsbad estimates its free pre-approved plans save about $8,000 to $16,000 in private design fees (City of Carlsbad).
If you are building in the City of San Diego and an accepted pre-approved plan fits your lot, you are looking at a materially different soft-cost line than the $36,000 modeled above. Check the current plan library before you hire an architect — not after.
The 750 square foot cliff
This is the most expensive line most San Diego homeowners cross without noticing.
Under California law, ADUs of 750 square feet or less are exempt from development impact fees. Above that, impact fees apply proportionally to the size of the primary dwelling. Several San Diego County cities layer their own under-750 waivers on top.
So the marginal cost of growing from 750 to 800 square feet is not 50 square feet of construction. It is:
| The last 50 sq ft | Cost |
|---|---|
| Construction at $430/sq ft | $21,500 |
| Newly triggered impact fees (our estimate) | $5,000 – $8,000 |
| Marginal cost of 50 sq ft | $26,500 – $29,500 |
| Effective rate on that space | $530 – $590 / sq ft |
Those 50 square feet cost about 25% to 35% more per foot than every square foot before them.
To be explicit: the $5,000–$8,000 impact-fee figure is our estimate, not a number taken from a published rate table — treat the marginal-cost range above as modeled, not quoted.
School fees are not on this list. They follow a separate rule: ADUs under 500 sq ft are exempt, and above that school districts may charge them (Ed. Code §17620; Gov. Code §66311.5(c)(3)). An 800 sq ft unit and a 750 sq ft unit both pay them, so crossing 750 only adds the district’s per-square-foot rate on the extra 50 sq ft.
That does not automatically make 750 the right answer. It does mean the decision to cross 750 should be a deliberate one, taken with the fee schedule open — not a last-minute “let’s round up to 800” on the drawings.
Coming Jan 1, 2027 — SB 1117: For a homeowner’s first two ADUs, impact fees will apply only to square footage above 750, calculated marginally — an 800 sq ft ADU would owe fees on 50 sq ft, not 800. The cliff math above reflects current 2026 rules. (Gov. Code §66311.5, as amended by SB 1117, Ch. 809, Stats. of 2026)
What moves the number most in San Diego
1. Which city. Covered above, and it is the largest single variable in this county. In Los Angeles the dominant variable is region. In San Diego it is jurisdiction.
2. Whether a pre-approved plan fits. Pre-approved plans accepted by the City of San Diego, Escondido’s free plan library, and similar programs elsewhere in the county. This is the only variable on this list that can move five figures before construction starts.
3. Coastal zone. Lots inside the Coastal Zone add a coastal development permit and review time. The drawings do not change; the calendar and the soft costs do.
4. Site slope and utility distance. Same as everywhere. A sloped lot adds retaining, engineered footings, and often a pump or crane for concrete. Distance to the sewer lateral and the main panel decides the trenching bill, and that figure barely moves with unit size.
5. Finish grade. $15,000 to $30,000 across a full unit between builder-grade and mid-grade.
What this number does not include
| Not included in $344,000 | Typical impact |
|---|---|
| Construction loan interest | Carried 9 to 14 months |
| Property tax reassessment | Only the new construction is added |
| Vacancy before first tenant | 1 to 3 months is common |
| Property management | 8% to 10% of rent |
| Contingency | 10% to 15% — budget it, don’t hope |
A 12% contingency on this build is $41,280.
If your lot is in the City of San Diego’s Bonus ADU Program, add one more recurring line most cost guides omit: a San Diego Housing Commission monitoring fee of roughly $150 per year per deed-restricted unit (SDHC ADU Bonus Program quick facts; confirm the current amount with SDHC), running the full 10 or 15 year term of the restriction.
→ Full year-one ownership cost: ADU all-in cost in California
→ Same 800 sq ft plan in the Bay Area, where the swing is impact fees rather than permits: San Jose ADU cost
Put your own quote in these six rows
We made a blank version of the six-phase ledger. Drop your contractor’s numbers into it and see which line doesn’t match ours.
Download the worksheet — CSV (import to Google Sheets), no email required
Compare any two builds side by side →
Want the regional benchmark ranges for each phase, updated quarterly?
Methodology and sources
Every figure on this page is a modeled median, not a quote. Here is exactly how it was built, so you can check it or reproduce it yourself.
| Input | Source |
|---|---|
| Statewide ADU rules, size limits, and the 750 sq ft impact-fee exemption | California HCD ADU Handbook |
| 2026 changes to State ADU Law | HCD Addendum, December 2025 |
| Declared construction valuations, City of San Diego | OpenDSD approval search |
| City of San Diego permit records and plan access | DSD Records |
| Permit volume and activity data | DSD Permit Activity Reports · San Diego Open Data Portal |
| City of San Diego permit fees | IB-501 Fee Schedule (Aug 2026) · ADU/JADU program page |
| County of San Diego permit fees | PDS-613 Building Construction Permit Fees (effective 10 Jul 2026) |
| Carlsbad ADU permit fees and pre-approved plans | City of Carlsbad ADU page |
| Encinitas ADU fee waiver | City of Encinitas ADU/JADU Guide (Jan 2026) |
| Escondido pre-approved plans and impact-fee exemption | Escondido PAADU program · Escondido 2025 Fee Guide |
| Bonus ADU monitoring fee | SDHC ADU Bonus Program quick facts |
| School fees on ADUs | Education Code §17620 · Gov. Code §66311.5 |
| Permit types filed after January 2018 | Accela Citizen Access — San Diego |
| Phase-level cost shares for residential construction | NAHB Cost of Construction Survey |
| Per-square-foot construction price benchmarks | NAHB Survey of Construction |
| Regulatory share of finished build cost | NAHB regulatory cost study, June 2026 |
On the four-city fee comparison. Permit fees are each jurisdiction’s posted building plan-check and inspection fees for an 800 sq ft detached ADU, checked 3 Oct 2026 and linked in the table. Soft-cost totals add the same $14,000 design and engineering line plus the impact, school, utility, and other fees from our model — shown in the “Other soft” column — which is why they are more than the permit and design columns combined. Fee schedules change annually and several cities revise mid-year. Pull the current schedule for your own jurisdiction before you budget — the comparison above is a shape, not a quote.
Check this number against your own address
- Open OpenDSD and search recent ADU approvals near your address, or use Permit Activity Reports for aggregate data.
- Note the declared construction valuation on comparable units.
- Compare it to the phase totals on this page.
Declared valuations run below contract price — treat them as a floor, not a total. Full detail on Methodology.
If your local numbers diverge meaningfully from ours, we want to know. See Corrections.
Our position
We don’t build ADUs. We don’t sell plans. We don’t sell your information to contractors. No figure on this site has been paid for or reviewed by anyone whose costs we publish. More on About.
Figures are San Diego County, 2026, stated as medians of contractor ranges. Costs vary by jurisdiction, site access, and finish level. This is construction cost information — not a quote, and not legal, engineering, or financial advice. Confirm requirements with your local building department and a licensed California contractor.
Frequently asked questions
How much does an ADU cost in San Diego in 2026?
About $344,000 for an 800 sq ft detached, single-story, new-construction unit with mid-grade finishes in the City of San Diego — roughly $430 per square foot. Our estimate for turnkey detached construction across San Diego County is roughly $375 to $600 per square foot, with the spread driven mostly by unit size, site conditions, and which jurisdiction issues the permit.
Why is San Diego more expensive than Los Angeles for the same ADU?
About 7% on the same unit, and in our estimate most of it is subcontractor rates in a smaller trade market rather than land or materials. Permit fees add the rest. The phase structure is nearly identical — systems and interior is 46% of the budget in both counties.
How much are ADU permit fees in San Diego?
It depends almost entirely on the jurisdiction. On an 800 sq ft detached unit, City of San Diego plan check and inspection run about $16,500 (IB-501), unincorporated County fees about $4,400 (PDS-613), Carlsbad about $2,000 to $4,000, and Encinitas waives its Development Services fees. That is a swing of about $16,500 on identical drawings.
Should I keep my ADU under 750 square feet?
It is worth modeling both. ADUs of 750 square feet or less are exempt from development impact fees under California law, and several San Diego County cities add their own waivers below that line. Crossing 750 means the marginal square footage effectively costs about $530 to $590 per foot once newly triggered impact fees are included (our estimate), against $430 for everything below it.
Does San Diego have free ADU plans?
Yes, in several jurisdictions. The City of San Diego accepts pre-approved plans from the County of San Diego, Chula Vista, and Encinitas programs, with a 30-day review. Escondido publishes four free pre-approved plans outright. Check the current library before hiring a designer — the savings disappear once you have already paid for custom drawings.
What is the San Diego Bonus ADU Program?
A city program that allows additional ADUs on qualifying lots in exchange for deed-restricting some units as affordable. It carries ongoing obligations most cost guides omit, including a San Diego Housing Commission monitoring fee of roughly $150 per year per restricted unit for the full 10 or 15 year term.