ADU Cost Breakdown: The San Francisco Homeowner's Reference for 2026

ADU Cost Breakdown: The San Francisco Homeowner’s Reference for 2026

We open with the number nobody puts on the first page of their website because they’re afraid it will scare you away: a fair, fully scoped, code-compliant ADU in San Francisco in 2026 runs about $430 to $680 per square foot. That’s the honest range. A 500-square-foot detached unit lands between $215,000 and $340,000. A garage conversion of the same size runs $175,000 to $260,000. These are real San Francisco numbers, not national averages that fall apart the moment you read the fine print about local labor rates and the city’s notoriously long permitting desk. In the next 4,000 words, we’ll explain exactly what sits behind those numbers, why they vary by more than $100,000, and how to spot the five line items where less scrupulous bidders bury their profit. For a broader overview, see our The Complete Guide to ADU in San Francisco.

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Quick Answer

An accessory dwelling unit (ADU) in San Francisco costs $430 to $680 per square foot for a detached new build and $350 to $520 per square foot for a garage conversion in 2026. A 500-square-foot unit therefore costs $215,000 to $340,000 (detached) or $175,000 to $260,000 (conversion). Permit fees, soft costs, and utility hookups add roughly $25,000 to $40,000 before construction begins, and SF-specific code requirements, soil conditions, and labor rates mean bids can swing $80,000 or more between contractors.

Table of Contents

• What Actually Drives the Cost of an ADU in San Francisco
• Line-Item Cost Ranges: Where Every Dollar Goes
• Garage Conversion vs. JADU vs. Detached ADU: The Per-Square-Foot Reality
• The Five Allowance Traps in SF ADU Contracts (and the Real Numbers to Replace Them)
• Soft Costs First-Time ADU Owners Miss
• How to Evaluate Competing Bids Without Getting Fooled by the Total
• What Makes San Francisco Different: Code, Soil, and Site Realities
• Common Mistakes to Avoid
• When to Call a Professional
• Frequently Asked Questions
• The Bottom Line

What Actually Drives the Cost of an ADU in San Francisco

You can ask five San Francisco contractors for a bid and get five numbers that differ by more than $100,000. None of them is crazy. Each one is making different assumptions about the same five cost drivers, and understanding those drivers is the difference between choosing a builder with confidence and hoping you picked the right envelope to open.

The first driver is whether you build new or convert. A garage conversion uses existing walls, a roof, and often a foundation, which can save $60,000 to $120,000 depending on the condition of the garage. But conversions have their own costs: seismic retrofits, raising the floor for plumbing, adding insulation to a structure never meant to be lived in. A Garage Conversion Contractor in San Francisco can walk you through whether your specific garage is a candidate or a money pit.

The second driver is site access. In neighborhoods like Glen Park, Twin Peaks, and parts of Noe Valley, narrow streets and hillside lots mean materials get hand-carried or crane-limited, adding 5 to 15 percent to framing and excavation line items. Flat, wide lots in the Sunset or the Richmond keep those costs flat too.

The third driver is soil. San Francisco has roughly 43 named soil types, and some of them, particularly the bay mud near Mission Bay and the serpentine formations in the central hills, trigger deeper foundations or even piering. A soils report costs $3,000 to $7,000 just to find out, and the foundation consequence can range from zero to $30,000.

The fourth driver is your choice of prefab versus on-site stick frame. Working with a brand like Mighty Buildings or Boxabl can shave months off the timeline and remove weather delays, but San Francisco’s fire-safety overlays and hillside siting requirements sometimes add back costs in crane delivery and material handling. It’s a trade-off, not a cheat code.

The fifth driver is finish level, and this is where the allowance trap lives. A contractor can bid honestly on the structure and still undershoot the finishes by $40,000. More on that below.

Line-Item Cost Ranges: Where Every Dollar Goes

Here is the breakdown we wish someone had handed us before we built our first San Francisco ADU. These are 2026 working numbers based on current SF subcontractor rates for a standard 500-square-foot detached unit with a modest finish level. Your specific site will move these, but the structure of the costs will not.

1. Permits and City Fees: $8,000 to $22,000

San Francisco’s Department of Building Inspection charges plan review fees, building permit fees, electrical, plumbing, and mechanical permits separately. For a deeper look at regulatory requirements, read ADU Permits, Codes & Inspections in CA: What You Need to Know. A typical ADU touches all of them. School impact fees are exempted statewide for ADUs under 750 square feet, which helps, but you’ll still face sewer connection review, PG&E service planning, and potentially a green building compliance fee. Budget $8,000 at the low end for a simple garage conversion and up to $22,000 for a detached unit on a hillside lot with additional review triggers.

2. Architectural Drawings and Engineering: $12,000 to $30,000

This includes measured as-builts of the existing conditions, architectural design, structural engineering, Title 24 energy compliance documentation, and sometimes a geotechnical review of existing foundation. Don’t let a contractor tell you this is “included for free.” Nothing is free. It’s either a line item or it’s buried in the markup.

3. Site Prep and Foundation: $28,000 to $75,000

For a detached unit, this means excavation, formwork, rebar, concrete, waterproofing, and drainage. In San Francisco, the foundation line item deserves particular scrutiny because of what is under your yard. Houses on bay mud, artificial fill, or steep lots can need helical piers or deep footings. A flat Outer Sunset lot on dune sand might need a standard 18-inch footing and nothing more. Same ADU, $40,000 difference in foundation cost.

4. Framing, Roofing, and Siding: $40,000 to $90,000

Lumber prices have stabilized since the pandemic spikes, but San Francisco labor has not. Framing carpenters in the city earn more than their counterparts almost anywhere else in the state, and the work is exacting because San Francisco has stricter seismic detailing than most jurisdictions. Roofing in the city runs $12 to $18 per square foot installed, and siding with a fire-resistant material like James Hardie fiber cement adds another $12 to $16 per square foot.

5. Mechanical, Electrical, Plumbing (MEP) Rough-In: $25,000 to $55,000

An ADU needs its own electrical subpanel, water heater, wastewater connection, and HVAC or heat pump. Getting sewer and water to a detached unit in a rear yard often means trenching through whatever is in the way, and that bidding line item is where surprises live. If the sewer lateral is shallow or the main is far from the unit, add $5,000 to $15,000 quickly.

6. Insulation, Drywall, and Title 24 Compliance: $18,000 to $32,000

California’s Title 24 energy code now requires solar-ready design for detached ADUs, high-efficiency heat pump water heaters, and tight insulation standards. The materials are not the expensive part. The testing and commissioning are. Budget specifically for blower-door testing and HVAC commissioning, which many low bids quietly omit.

7. Finishes: Kitchen, Bath, Flooring, Cabinets, and Paint: $45,000 to $95,000

This is the line where almost every first-time ADU owner learns what a kitchen actually costs. In San Francisco, a code-compliant kitchen in a new ADU, meaning one with proper ventilation, a gas or induction range, a dishwasher with its own circuit, and cabinets that will survive longer than the guarantee period, realistically runs $18,000 to $28,000 for the kitchen alone. A full bathroom with tile, a vanity, a shower, and proper waterproofing runs $14,000 to $24,000. Engineered hardwood or LVP flooring adds $8 to $14 per square foot installed.

Quick reference table for a 500-square-foot detached ADU, San Francisco 2026:

• Permits and fees: $8,000 to $22,000
• Design and engineering: $12,000 to $30,000
• Site prep and foundation: $28,000 to $75,000
• Framing, roofing, siding: $40,000 to $90,000
• MEP rough-in: $25,000 to $55,000
• Insulation, drywall, Title 24: $18,000 to $32,000
• Finishes: $45,000 to $95,000
• Total: $176,000 to $399,000 (before contingency)

That range is honest. The difference between the low end and the high end is site conditions, access, finish level, and whether you choose prefab or stick-built. Every one of those variables is knowable before you sign.

Garage Conversion vs. JADU vs. Detached ADU: The Per-Square-Foot Reality

San Francisco allows three distinct ways to add a second unit to a single-family lot, and they are not the same product, not the same cost, and not the same timeline.

Garage Conversion

A garage conversion, also called converting an existing garage into a dwelling, is the cheapest path in most cases because the shell already exists. You are not paying for new framing, a new roof, or a new foundation in most cases. What you are paying for is bringing a building never meant for human habitation up to a standard where people sleep in it: flood protection at the bottom of exterior walls, seismic bracing of the existing structure, insulation, new windows, and a completely new interior. In our experience, a San Francisco garage conversion runs $350 to $520 per square foot, so a 400-square-foot conversion lands between $140,000 and $208,000.

But there is a caveat specific to San Francisco: many of the city’s garages predate the 1940s, and their foundations are not meant to support a finished living space above their original concrete. Watch for red flags with our ADU Warning Signs: A San Francisco Homeowner’s Reference Guide. If your garage is unpermitted, has structural issues, or has a floor that sits below grade, the conversion can start to look like a knockdown and rebuild, and the “cheap” option stops being cheap. A garage conversion contractor can tell you quickly where your structure falls.

JADU (Junior ADU)

A JADU, or junior accessory dwelling unit, is a unit created within the existing footprint of a single-family home, with a 500-square-foot maximum and often a shared bathroom allowance. JADUs are the least expensive per square foot, typically $250 to $400, because they ride on existing systems and don’t require a separate utility connection. But they are also the most constrained: the unit cannot be sold separately, and the owner is typically required to live on the property in one of the two units. This is a good option for families in the Richmond or Sunset who want a unit for an aging parent or adult child without adding a new structure in the yard. ADU Builders in San Francisco can help you decide if a JADU works for your specific floor plan.

Detached ADU

A detached ADU is a completely new structure in the rear yard, and it is the most flexible and the most expensive at $430 to $680 per square foot. You get full independence, separate utility metering, the ability to rent to a non-family member at market rates, and the option to sell the property later with a unit that appraises as a true second dwelling. For many San Francisco homeowners, the appraisal value and rental income justify the higher upfront cost. Others choose a prefab unit from Plant Prefab or Tuff Shed’s studio line to accelerate the timeline, though even prefab units need local site work, foundation, and utility connections.

Per square foot, a detached ADU costs about 20 to 30 percent more than a garage conversion. Per dollar of added property value, it often returns more because it appraises as new square footage rather than a remodeled existing garage.

The Five Allowance Traps in SF ADU Contracts (and the Real Numbers to Replace Them)

An allowance in a construction contract is a placeholder dollar amount for work the contractor has not yet priced with a specific supplier. The contract says the kitchen has an $8,000 allowance, but the actual kitchen costs $22,000, and the difference becomes a change order you did not budget for. This is not illegal. In many cases it is standard practice. But it is the single largest source of the “our ADU went $60,000 over” story San Francisco homeowners tell.

Here are the five allowance traps we see most often in bids around the city:

1. Kitchen finish allowance set at $8,000 to $12,000. The realistic number for a code-compliant kitchen in a new San Francisco ADU, with cabinets, counters, ventilation, and appliances that meet current energy standards, is $18,000 to $28,000. Take the contract’s allowance and add $10,000 minimum.
2. Electrical allowance that assumes no subpanel upgrade. Many San Francisco homes have service panels from the 1960s and 1970s. An ADU generally requires a new subpanel, and if your main panel lacks capacity, a main panel upgrade, which can run $4,000 to $8,000 in the city. If the allowance does not mention the main panel, it will be a surprise.
3. Site work allowance of “zero.” Some bids treat the rear yard as a blank, flat canvas. It rarely is. In neighborhoods like Bernal Heights or Potrero Hill, the rear yard often slopes, has an old concrete pad from a demolished garage, or has overgrown vegetation. Demolition, grading, and debris hauling can add $10,000 to $30,000 that never appears in a bare-bones bid.
4. Finish flooring allowance of $3 per square foot. At that price, you are choosing from the least durable products on the market. A floor that survives tenants, pets, and San Francisco’s fog-season moisture needs $8 to $14 per square foot installed. The trap is a change order for the difference.
5. Zero included for utility connection fees. PG&E charges for new service connections, and San Francisco Public Utilities Commission charges sewer lateral connection and capacity fees. These are not optional and not the contractor’s markup. Expect $3,000 to $10,000 depending on your location and the distance from existing infrastructure.

Our approach at Lodgewick ADU San Francisco is to write the real number the first time. Every bid we deliver has allowances marked clearly, with a note explaining what the allowance includes and what would move it. You should demand the same from any builder you interview. Also ask what happens if the allowance is not fully used: some companies keep the difference as profit. We return it to the project, line item by line item, in writing.

Soft Costs First-Time ADU Owners Miss

“Soft costs” is the industry term for everything you pay before a single shovel hits the dirt. In San Francisco, these costs are higher than almost anywhere else in the state, and most ADU cost articles pretend they don’t exist. Here they are, with real numbers.

Architectural Drawings: $6,000 to $18,000

San Francisco requires a full set of drawings for any ADU, including site plans, elevations, sections, and often a survey showing exact property lines. Some homeowners use a design-build firm that includes drawings in the contract. Others hire an architect separately. Either way, the work is real and it is not optional.

Structural Engineering: $3,000 to $8,000

Because San Francisco sits in Seismic Design Category D or E depending on location, structural engineering is not a “nice to have.” Your engineer calculates the foundation, shear walls, and roof connections to the appropriate seismic standard. This is not the place to save money.

Soils Report: $3,000 to $7,000

A geotechnical engineer drills or probes your rear yard to determine what is under the grass. In the Marina, you’re on artificial fill that can settle and liquefy in an earthquake. In the Sunset, you’re on dune sand that performs better than most people expect. On Twin Peaks, you might hit bedrock ten feet down. The foundation design depends entirely on what the report says, and skipping this step means guessing on the single most expensive structural component.

Title 24 Documentation: $1,500 to $3,500

California requires a Title 24 report showing your ADU meets minimum energy standards. This is a separate document from the building permit and usually prepared by a specialized consultant.

Utility Connection Fees: $3,000 to $10,000

PG&E charges to bring a new electrical service to a detached ADU, and SFPUC charges sewer connection fees. The exact number depends on site location and the age of existing infrastructure. Budget for the upper third of the range unless your contractor can document otherwise.

Temporary Power and Site Protection: $2,000 to $5,000

During construction, the crew needs temporary power and weather protection. If you are building in the wet months from November through March, this number goes up because San Francisco’s rainy season demands covered work areas and additional erosion control.

If you’ve been adding along, these soft costs total $18,500 to $51,500 before construction begins. Any bid that does not show them as line items is either burying them in the main construction number or planning to add them as change orders later.

How to Evaluate Competing Bids Without Getting Fooled by the Total

Comparing two ADU bids by the bottom-line number only is like comparing two houses by their square footage only. The cheaper one might be cheaper because it includes less.

Here is a step-by-step process for evaluating three or four bids side by side:

1. Standardize the scope. Write down the square footage, the number of bedrooms and bathrooms, the finish level (basic, mid, or high), the structural approach (stick-built, panelized, or prefab from a brand like Mighty Buildings or Plant Prefab), and the expected permit path. Apply those same parameters to every bid.
2. Create a line-item comparison table. List each phase: permits, design, site work, foundation, framing, roofing, siding, MEP, insulation, drywall, finishes, and contingency. For each bid, note whether the line is present, absent, or an allowance. A missing line item is a future change order.
3. Check the exclusions list. Every legitimate bid has an “exclusions” section. Read it carefully. If one bid excludes the sewer lateral connection and another includes it, the totals are not comparable.
4. Ask what contingency they carry. Most good builders carry a 10 percent construction contingency for unknown conditions. A bid that carries none is either optimistic or planning to pass surprises to you immediately.
5. Ask for references from the last three jobs in San Francisco. Specifically ask for jobs in your part of the city. A builder who routinely works in the Outer Richmond may be less practiced with the hillside access challenges of Diamond Heights. A home addition contractor who does a lot of work in your neighborhood already knows the inspectors, the soil, and the parking constraints.
6. Verify what the written guarantee covers. Ask for the warranty terms in writing before you sign. At Lodgewick, the 365-Day Done Right Promise is written into every contract. If the work is not done right, we come back and make it right. That promise is not a verbal assurance. It’s a line in the document.

The goal of this exercise is not to choose the cheapest bid. It is to make the hidden costs visible so you can choose the bid that actually reflects the work.

What Makes San Francisco Different: Code, Soil, and Site Realities

San Francisco is not Sacramento. It is not Sacramento in terms of labor costs, permit timelines, soil conditions, or seismic requirements, and anyone who gives you a number based on a statewide average is guessing.

The DBI permit timeline for an ADU in San Francisco is longer than the state mandate would suggest. The state requires no more than 60 days for an ADU permit decision, but that clock runs after the application is accepted, and pre-acceptance review in San Francisco can add months. A realistic total from “plans submitted” to “permit issued” is 6 to 10 months in the current environment. That timeline is a soft cost, especially if you are paying a mortgage on the property while waiting. For more guides & resources on planning your project, explore our blog.

Seismic requirements drive framing costs. San Francisco is in a high-seismic zone, and the building code requires fully sheathed shear walls, hold-downs at every post, and a continuous load path from roof to foundation. A contractor who builds ADUs in the East Bay for $400 per square foot might be unable to deliver the same structure for that price in San Francisco because the same building on this side of the bay needs more engineering and more hardware.

San Francisco’s cool, foggy, marine climate also shapes the material list. Moisture-resistant siding like the James Hardie fiber cement line performs better than wood siding in the fog belt of the Sunset and Richmond. The roof needs to be installed for a building that sees zero degree-days of annual snow but a great deal of horizontal moisture. These are San Francisco-specific decisions that a national cost calculator does not know to make.

And then there is parking. If your project removes a covered parking space, San Francisco historically required replacement parking unless the project qualifies for an ADU exemption. Recent state laws have relaxed this for ADUs, but exact rules depend on your lot’s zoning and historic status. Work this out with your designer before you fall in love with a plan.

Common Mistakes to Avoid

• Trusting an allowance-based bid without asking for the backup numbers. If the contract says “kitchen allowance: $10,000,” ask for a line-item estimate of the actual kitchen. If they can’t produce one, the allowance is a guess, and you will pay the difference later.
• Starting construction before Title 24 and permit documentation is complete. In San Francisco, the order of operations matters. Starting work before the permit is issued triggers stop-work orders, fines, and sometimes a forced re-review of the entire project. The risk is real, and it is expensive.
• Choosing a builder on bottom-line price alone. A $50,000 cheaper bid that is missing several scopes will cost $60,000 more by the end. Compare scope first, price second. Always.
• Overlooking access constraints on your own lot. Measure the width of the path from the street to the rear yard. If it is narrower than 4 feet, you need a crew that has dealt with narrow-access sites before, and your material handling costs will be higher. Knowing this upfront prevents a mid-project surprise.
• Skipping the soils report to save $5,000. In San Francisco, the cost of a soil problem discovered during excavation can be five to ten times the cost of the report. On bay mud or fill, it can be more.
• Believing “prefab is always cheaper.” A prefab unit from Boxabl or a Tuff Shed studio line can save time and reduce weather exposure, but in San Francisco the foundation, utility, and site work are often the same cost as stick-built. Prefab changes where the money goes, not always how much.
• Hiring a general contractor unfamiliar with San Francisco’s DBI process. An experienced SF builder knows which plans pass review the first time and which inspectors need which details. Inexperience costs time, and time is a real cost in a city where carrying costs are high.

When to Call a Professional

You should call an Home Addition Contractor in San Francisco early, before you commit to a floor plan or a budget spreadsheet. The right time to bring in a builder is after you have a rough idea of your goals but before you have made decisions that are expensive to undo. A good builder will tell you whether your garage is structurally convertible, whether your rear yard can support a detached unit without heroic foundation work, and what your specific permit path will look like. All of that information changes your budget before you have spent a dollar on drawings.

Lodgewick ADU San Francisco offers free estimates in San Francisco, and our process starts with a site visit and a written scope review, not with a sales pitch. Call (707) 760-0502 to schedule a free estimate and get a written price before any work begins. Every Lodgewick project carries The Kingswell Guarantee: a written price first, a defined arrival window, a clean job site, and a written guarantee with a free return visit if the work fails within the 365-day guarantee period.

Frequently Asked Questions

The Bottom Line

The most useful thing you can do before committing to a San Francisco ADU is to understand your costs at the line-item level. Know what the permit desk will charge, what your soil is likely to do, what a fair kitchen costs, and what allowances in a contract are actually placeholders for future surprises. Then choose a builder who writes the real number, puts the guarantee on paper, and commits to coming back if something goes wrong. Lodgewick ADU San Francisco has been doing exactly that since 2010, and we’re happy to walk your lot, explain your options, and give you a written estimate. Call (707) 760-0502 to schedule your free estimate. No pressure, no guessing. Just a number you can plan around.

Written by Russ Delmar, Owner at Lodgewick ADU San Francisco, serving San Francisco since 2010.