Accessory dwelling units and garage conversions turn existing or new space into a separate place to live. The cost depends heavily on how much of the structure already exists.
What the permit filings show
Based on 6,907 residential building-permit filings. The median adu / garage conversion valuation runs from $60,000 in San Francisco to $218,794 in Seattle, about 3.6× the cheapest city. Within a city the spread matters too: the middle half of San Francisco filings spans 223% of its median, versus 87% in Austin.
CityTypical range (P25–P75)MedianFilings
San Francisco$16,445 – $150,000$60,000189
Los Angeles$35,000 – $150,000$77,9203,077
Chicago$65,145 – $200,000$120,00062
Austin$80,000 – $200,000$138,000379
Seattle$143,327 – $500,000$218,7943,200
Filings are declared valuations from each city's open permit dataset, limited to residential records classified as adu / garage conversion work and valued between $1,000 and $2,000,000. A city is listed only when at least 30 filings qualify. Full methodology. Data refreshed September 2026.
What the numbers say
Sample size varies by city too — 3,200 qualifying adu / garage conversion filings back the Seattle figures above, versus just 62 for Chicago — and while both clear the site's 30-filing minimum, Seattle's far larger sample settles into a steadier shape than Chicago's, which sits closer to that threshold.
Zoom out across every city on this page and the legitimate range is wider still: the lowest 25th-percentile figure is $16,445, in San Francisco, and the highest 75th-percentile figure is $500,000, in Seattle — a 30.4× gap that reflects different cities, codes and building stock, not overcharging.
In Austin, quotes cluster tightly: the middle half of filings sits between $80,000 and $200,000, so a quote near the $138,000 median there is the safer assumption. In San Francisco, that band runs from $16,445 to $150,000 — wide enough that a quote toward either edge is still plausible, so treat San Francisco's median as a starting point for a conversation, not a target.
Conversion versus new build
Converting a suitable garage reuses foundations and framing, so the work centers on insulation, fire separation, services and finishes. A detached new-build ADU needs foundations, structure and utility connections, and costs correspondingly more. Filings labeled ADU include both.
Utilities and approvals
Separate meters, sewer connections and fire-safety requirements can be significant and vary by jurisdiction. Confirm the permit path and occupancy requirements with the local authority before committing to a design.
Checking feasibility first
Have the existing structure assessed before pricing finishes. Discovering that a garage slab or roof needs major work after design is complete can change the economics of the whole project.
What drives the cost of an ADU or garage conversion
Whether you're converting an existing structure or building new sets the scale of the whole project: a garage conversion reuses foundations, framing and a roof that already exist, while a detached new-build ADU needs all of that from scratch.
Utility connections are a major and easy-to-underestimate cost. A separate water, sewer or electrical meter, or simply extending existing service to a detached structure, can require trenching and utility-company coordination well beyond the structure itself.
Fire separation, egress windows and accessibility requirements for a legal, occupiable dwelling add cost that a simple storage conversion would not need. Kitchen and bathroom work, where included, prices the same way it would in any home: plumbing runs, ventilation and finish level all move the cost independently of the structure around them.
A worked example
Take Seattle, the city with the most filings behind its figures (3,200). Suppose two contractors quote for the same adu / garage conversion: one at about $218,794, in line with the local median, and another at about $600,000, well above the local P75 of $500,000.
Neither figure alone says which contractor is right: $218,794 sits where most Seattle filings land, a reasonable opening point, while $600,000 sits above where three-quarters of them do — not necessarily a red flag, since a new-build structure rather than a conversion, a separate utility connection or added fire-safety work can all justify it — but a scope worth having spelled out before you sign. Both figures are illustrative.
What an ADU permit valuation leaves out
The filed valuation covers the permitted scope of the conversion or new build — structure, utilities, fire safety and finishes as declared. It is not automatically the final amount paid to a contractor.
Site work such as grading, a new driveway or path to a detached unit, and landscaping disturbed by construction are commonly outside the core ADU permit and priced separately.
Furnishing the unit, and any impact or connection fees charged by the utility or the local authority rather than the contractor, typically sit outside both a contractor's quote and the declared construction valuation alike.
Use your quote to test the range
What this project can cover
An ADU or garage conversion can include habitable-space upgrades, insulation, fire separation, windows, services, kitchen or bathroom work, and finishes.
What may sit outside it
New utility meters, sewer upgrades, site work, demolition, furnishings, and major foundation or roof reconstruction may not be included.
Checks before you accept a quote
Confirm the existing structure is suitable before pricing finishes.
Break out utility, fire-safety, and accessibility work.
Check the permit path and occupancy requirements with the local authority.
Questions to ask before you sign
Is the existing structure — if converting a garage — being assessed for foundation and roof condition before finishes are priced?
Does the quote include a separate utility meter, or is the unit sharing the main house's service?
What is included for fire separation, egress and any accessibility requirements the permit will require?
Is site work — grading, or a path or driveway to the unit — included or separate?
What occupancy or rental restrictions apply to this permit path, and who is confirming that with the local authority?
How is a kitchen or bathroom priced if one is being added where none existed?
What happens to the price if the existing structure needs more foundation or framing work than expected?
Questions people ask
Is a garage conversion cheaper than a new-build ADU?
Generally yes, because a conversion reuses existing foundations, framing and a roof. A detached new-build ADU needs all of that built new, which is a large part of why new-build ADU costs tend to sit higher in the published range.
Does this include getting a separate address or utility meter?
Only if that work was part of the declared permitted scope. Separate utility connections and addressing are handled by the utility and local authority and are commonly priced and processed outside the core construction permit.
Can I rent out an ADU built under this kind of permit?
That depends entirely on local zoning and the specific permit path used, not on anything in this dataset. Confirm occupancy and rental rules with the local planning or building department before committing to a design.
Why does the typical cost vary so much between cities for what sounds like the same project?
Local zoning, utility requirements, and how much of an existing structure can be reused all vary by city, and each one moves the price independently of labor rates.