How Much Does an ADU Cost in San Diego? (2026 Numbers)

Ask ten contractors what an ADU costs and you'll get ten vague answers. Here are ours, from projects we have actually built across San Diego County. No teaser pricing, no "it depends" hand-waving: real ranges, what sits inside them, and the property-specific items that move them.

A finished Nucave attached ADU. Every number in this guide comes from projects like this one.

The short answer

For a detached, new-construction ADU in 2026, we see projects land between roughly $385 and $475 per square foot of finished space. Smaller units sit at the top of that range and larger units at the bottom, because every ADU carries the same fixed costs: a kitchen, a bathroom, utility connections, permits, and mobilization. Spread those over 400 square feet and the rate is high; spread them over 1,200 square feet and it drops.

Here is how that plays out by size, using the same numbers we quote from:

ADU size	Approximate rate	Approximate core cost
400 sq ft	~$475 / sq ft	~$190,000
600 sq ft	~$435 / sq ft	~$261,000
750 sq ft	~$415 / sq ft	~$311,000
1,000 sq ft	~$390 / sq ft	~$390,000
1,200 sq ft	~$385 / sq ft	~$462,000

Two things to notice. First, the price per square foot falls as the unit grows, so the middle sizes are often the value sweet spot. Second, doubling the size does not double the cost: going from 600 to 1,200 square feet adds about 77 percent to the core budget while doubling your rentable space. That math is why so many of our clients land between 750 and 1,200 square feet.

A few real-world anchors from our own recent projects:

• A 1,000 sq ft detached ADU with 3 bedrooms and 2 baths in Vista: around $390 per square foot for the core build.
• A 587 sq ft garage conversion in Vista: in the $185,000 to $200,000 range.
• Garage conversions overall: roughly $150,000 to $230,000 depending on size and condition, because the structure already exists. You can see both types finished among our built projects.

What's inside the core number

When we quote a core construction range, it covers the complete, turnkey unit: foundation, framing, roofing, windows and doors, full kitchen and bathroom, electrical, plumbing, HVAC, drywall, flooring, paint, and finish carpentry. It is a home someone can move into, not a shell.

Finish level moves the number too. A rental-grade finish package trims a few percent off; a premium package with custom cabinetry, upgraded fixtures, and designer finishes can add 10 to 20 percent. On a $390,000 core build, that finish decision alone is a swing of up to $78,000, which is why our ADU Project Planner lets you flip between packages and watch the range change.

What's on top of construction

Two costs many homeowners forget to budget:

• Plans, engineering, and permit processing: typically $10,000 to $18,000. This covers architectural drawings, structural engineering, energy calculations, and shepherding the application through the city.
• City and agency fees: typically $8,000 to $18,000, varying by jurisdiction. School district fees can apply on top in some areas.

That second line deserves a closer look, because where you live changes it dramatically.

Not building detached? How the other types price

The $385 to $475 per square foot curve above is for detached new construction, the most common request. The other configurations price off that same curve:

• Attached ADUs price off the same curve as detached. Sharing a wall and nearby utilities can simplify parts of the build, but configuration and site conditions usually decide the final number, so we quote them the same way and let your lot tell us the rest.
• ADUs above an existing garage run 10 to 20 percent more than the detached curve. You are building on top of a structure that was never designed to hold a home, so structural upgrades and fire separation do the damage.
• A new garage with an ADU above adds a fixed $125,000 to $165,000 garage allowance on top of the ADU itself. It is the most expensive path per square foot, and also the only one that adds parking while adding a home. One of our San Diego builds took exactly this shape: a 504 sq ft one-bedroom over a new garage, with a wraparound balcony.
• Garage conversions ignore the curve entirely and price as a band: roughly $150,000 to $230,000 depending on size and the condition of the structure.
• Decks and balconies price at $125 to $225 per square foot of deck. A 150 sq ft covered deck adds roughly $19,000 to $34,000, and elevated balcony and stair packages carry their own premium.

Where the money goes over the project

Understanding the price is easier when you see the sequence. Every Nucave project moves through the same four phases:

1. Feasibility. We check your lot, zoning, utilities, and access before you spend a dollar on design. This is where the property-specific list above gets answered, and it is the single highest-leverage step in the whole budget.
2. Design. Plans shaped around your property and how you will use the space, drawn by the same team that builds it. The $10,000 to $18,000 plans and engineering line lives here.
3. Permits. We submit, track, and clear city corrections. State law gives cities 60 days to decide on a complete ADU application, and the city fee line gets paid here.
4. Build. One accountable team from trenching to final inspection. The core construction number, and any property-specific work, is spent here.

The order matters because money committed early constrains everything after. A thorough feasibility pass is why our projects rarely meet a mid-build surprise that a site walk would have caught.

Fees are a city-by-city game

San Diego County cities have real differences in what they charge, and knowing them before you buy plans is free money:

• Oceanside waives every city development impact fee on ADUs of any size, one of the most generous policies in the county.
• The City of San Diego exempts the first two ADUs on a property from development impact fees regardless of size, and any ADU under 750 sq ft is exempt everywhere in the city.
• Escondido exempts ADUs under 750 sq ft and, per its current fee guide, waives wastewater, water, traffic, and park impact fees, and it offers four free pre-approved ADU plans that skip most design costs.
• Carlsbad estimates $2,000 to $4,000 in permitting fees for most ADUs, with impact fees waived under 750 sq ft.
• Encinitas, Vista, and San Marcos each have their own quirks, from coastal permit layers to setback details, covered in our city guides.

The pattern worth remembering: staying under 750 square feet avoids impact fees almost everywhere, thanks to state law. If your budget is tight, that threshold is a real design constraint worth respecting.

A Nucave conversion in Encinitas, finished. This one came to us stalled mid-project and needed re-engineering to keep its original permit. Conversions are the budget path, but not always the easy path.

What moves the price on YOUR lot

The biggest cost surprises are property-specific, not construction-specific. These are the allowance ranges we carry when a condition applies:

• Electrical service upgrades: $5,500 to $25,000. Many older homes run 100-amp panels that can't carry a second dwelling.
• Water meter or service upgrades: $3,500 to $40,000. The wide range is real; a meter swap is cheap, a new service line under landscaping is not.
• Long utility runs to a back corner of the lot: $15,000 to $40,000. Every foot of trench between your house connections and the ADU costs money.
• Work in the public street to connect utilities: $30,000 to $60,000. Cutting asphalt in the public right of way brings traffic control, inspections, and restoration.
• Bad soils or special foundations: $10,000 to $50,000.
• Retaining and grading on sloped lots: $15,000 to $100,000. Slope is the single biggest wildcard in backyard construction.
• A boundary or topographic survey when property lines are uncertain: $2,500 to $7,500.
• Enhanced fire-rated construction near property lines or in fire hazard zones: $5,000 to $25,000.
• Tight construction access: $5,000 to $30,000. If a mini excavator can't reach the build site, work slows down.
• Demolition, new driveways, and drainage corrections each carry their own ranges.

The part nobody photographs for Instagram: the utility trench. Distance from your house to the ADU is one of the biggest hidden cost drivers.

This is why two identical ADUs on different lots can be $80,000 apart, and why any quote that skips these questions is not a real quote. When we run your numbers, anything we can't verify gets flagged as an open item instead of silently padded into your price, so you always know what is real and what still needs checking.

A worked example

Say you want a 750 sq ft detached one-bedroom in Vista with a standard finish. The core build lands around $311,000. Add plans and engineering ($10,000 to $18,000) and city fees ($8,000 to $18,000), and the planning budget is roughly $330,000 to $350,000 before property-specific items. Flat lot with good access and utilities nearby? You are done. A sloped corner with a 100-amp panel and a long sewer run? Add $30,000 to $80,000, and better to know now than at plan check.

How to keep the number down

Honest levers, in rough order of impact:

1. Convert instead of building new. If you have a garage you can spare, conversion is the budget path.
2. Stay under 750 sq ft to dodge impact fees nearly everywhere.
3. Pick a proven layout. Our floor plans start from layouts we have built before, and both Escondido and the County of San Diego offer free pre-approved plans.
4. Keep the ADU close to existing utilities. Every foot of trench is money.
5. Choose finishes for the mission. A rental unit does not need the premium package; a well-planned rental ADU can pay for itself.

How do people pay for this?

Most of our clients do not have $350,000 in savings, and they should not need it. Home equity lines, home equity loans, cash-out refinances, and renovation loans that borrow against your home's after-ADU value are the common routes. We are builders, not lenders, so we keep an honest financing options guide that explains each path without selling you one.

Frequently asked questions

What does an ADU cost per square foot in San Diego? Roughly $385 to $475 per square foot for detached new construction in 2026, depending mostly on size. Conversions price differently because the structure exists.

What is the cheapest way to add an ADU? A garage conversion, typically $150,000 to $230,000 all-in for the construction. Whether it beats a detached build on your lot is a free consultation away.

Are online ADU calculators accurate? Most are not, because they ignore your property. Ours is built from our own project data and separates verified costs from open questions; try the Nucave ADU Project Planner and see the difference.

Does an ADU add value to my home? ADUs add livable square footage and rental income potential; county assessors value the addition without reassessing your whole property. The investment math depends on build cost, financing, and local rents, which is exactly what a feasibility conversation covers.

How long does the whole process take? Cities must approve a complete ADU application within 60 days by state law, and design comes before that. Construction time depends on the project, which is one more reason to start with real numbers instead of guesses.

What does an above-garage ADU cost? Expect 10 to 20 percent over the detached curve when building on an existing garage, or the ADU cost plus a $125,000 to $165,000 garage allowance when building a new garage with the unit on top.

Do I pay for an estimate? No. The Project Planner is free and instant, and the property review consultation that follows is free too. You pay when you hire us to design and build.

Get your number, not the county average

Our ADU Project Planner walks you through the same questions we ask on a site visit and gives you a planning range built from our real project data, in about two minutes. Anything you can't answer gets flagged for verification instead of padded into your price. When you want human eyes on it, book a free consultation and we'll walk your property together.