| Quick Answer
Four ADU routes exist in Contra Costa County: a junior ADU inside the house, a garage conversion, an attached unit, or a detached new build. Under 2026 state law a city must decide whether your application is complete within fifteen days and approve or deny a complete application within sixty. Both Concord and Martinez run pre-approved plan programmes that cut review time and fees. |
The four routes, and what actually separates them
The decision is rarely about the building. It is about what your lot already gives you a slab, walls, a roof, and services close by versus what has to be built from nothing.
| Route | What already exists | What has to be built | Usually best when |
|---|---|---|---|
| JADU, inside the house | Everything structural | A kitchen, an entry, sometimes a bathroom | You have a spare room or a wing that can be separated, and you want the cheapest route |
| Garage conversion | Slab, walls, roof | New entry wall, insulation, level floor, full interior, services | The garage is sound, tall enough and near the panel and sewer |
| Attached ADU | One shared wall, often one service run | Foundation, structure, roof tie-in, interior | The side of the house has room and the roofline can be tied cleanly |
| Detached new build | Nothing | All of it, plus trenched services and site access | You have the garden to give and want a genuinely separate dwelling |
The thing that surprises most people: a detached unit at the back of a deep lot costs more than an identical unit near the house, and the difference is almost entirely trenching and site access rather than the building itself.
What changed in 2026
- A fifteen-day completeness determination. The local agency must decide whether your application is complete within fifteen days. If it misses that window, the application is deemed complete. Applicants also gained a formal right to appeal a finding of incompleteness, which matters because incompleteness findings were previously the main way a project could stall indefinitely.
- The sixty-day clock still applies. A complete ADU application must be approved or denied within sixty days.
- Size is no longer capped by lot coverage. ADUs up to 1,200 square feet are allowed regardless of lot size, which removed the rule that previously limited what smaller lots could build.
- Impact fee exemptions continue. ADUs of 750 square feet or less remain exempt from most development impact fees, and the smallest units are exempt from school fees. This is a large part of why so many ADUs are designed at just under 750 square feet.
- Reduced setbacks and increased height allowances for detached units, which makes two-storey and above-garage designs viable on more lots than before.
The pre-approved plan programmes nobody mentions
This is the most useful thing in this article and it is barely known.
Both the City of Concord and the City of Martinez publish pre-approved ADU plan sets. You download an architectural plan, structural plan and Title 24 energy forms that the city has already reviewed and approved. You still need a site plan, a grading plan if applicable, and your own contractor but the plan review you submit is dramatically narrower, because only the site-specific elements need checking.
What that means in practice: expedited review, reduced permit fees and reduced plan check fees, charged hourly for the site plan review only rather than for the whole set.
The catch, and it is worth saying plainly: if you modify the pre-approved plans, the application is subject to additional review and additional fees. The programme rewards taking the plan as drawn. If you want a bespoke unit, take the normal route and budget for it properly.
Concord states that ADU permits are typically issued within sixty calendar days of a complete application, and that pre-approved ADU permits are issued considerably faster because only the site plan requires review. ADUs and JADUs in Concord are reviewed through the building permit process and do not require a public review process, which means that if the unit complies with the standards, staff approve it.
City by city, what to check locally
| City | What to confirm before you design |
|---|---|
| Concord | Pre-approved ADU plan programme available. Applications go through the Virtual Permit Center. The city publishes a plan review turn-around schedule, and expedited review is available for a fee though it typically reduces only the Building division’s time, not the other divisions’. Floodplain projects add roughly a week. |
| Martinez | Pre-approved ADU plan programme available. As of 1 September 2026 the Building Division is fully electronic paper applications and paper plan sets are no longer accepted unless the Chief Building Official grants an exception. Contractors applying online need a current City of Martinez business licence. |
| Pleasant Hill | Applications and resubmittals go through the PHill Online Service Center. Fifty percent of the permit fee is invoiced before plan review begins, so a delay in paying is a delay in reviewing. |
| Walnut Creek | All building permits are online only no paper, no email, no counter issuance. Note that hourly plan check fees begin after the third round of building division review, which is a direct financial incentive to submit complete. |
| Lafayette | Hillside and tree protection considerations affect a large share of lots and frequently drive the design more than the ADU rules do. |
| Unincorporated county | The county ordinance applies. Confirm current size and height limits before designing. |
The twenty-minute walk around your property
Before you commission a design, go outside and answer these seven. Any one of them can move a budget by tens of thousands, and all seven are free to check today.
- Where is the electrical panel, what is its amperage, and how far is it from where the unit would sit? Many Contra Costa homes of a certain age have 100 amp service, which may not carry an additional dwelling with modern loads. A service upgrade involves the utility and runs on the utility’s timeline.
- Where is the sewer clean-out, and is it downhill from the proposed unit? Waste runs on gravity. A unit uphill of the connection needs a pump, which is a cost and a maintenance item.
- Can a small excavator physically reach the build area? Measure the narrowest point of the side access. If the answer is a wheelbarrow, the foundation labour changes substantially.
- If a garage conversion is on the table, what is the height from the slab to the underside of the lowest joist or truss? Measure to the lowest structural member, not the ridge, then allow for the insulated ceiling assembly and any floor build-up.
- How close would the unit sit to the property lines, and are there protected trees in the footprint?
- Is the lot flat, or will it need retaining or grading?
- Does an association govern the property, and what design standards does it enforce?
Photograph the answers. A contractor who sees those seven photographs can tell you more in ten minutes than they could from a floor plan.
What it costs
An ADU’s cost per square foot is higher on small units than large ones, because a kitchen, a bathroom, a service connection and a permit set cost much the same whether the unit is 400 square feet or 900. That is why “cost per square foot” is the number people ask for and the number that misleads them most.
Rather than publish a table that will be stale by spring, use the estimator. It takes ADU as a project type, asks the size and the finish level Essential, Signature or Bespoke and asks which of six complications apply, including foundation work and electrical panel upgrade, both of which are common on ADU projects. It returns a planning range in about thirty seconds with no email required.
A planning range assumes demolition and haul-off, permits and plan check for one jurisdiction, electrical and plumbing to current code, materials at your finish level, and project management through to final clean. It cannot yet know what is behind your walls, whether the panel carries the new load, the soil and drainage condition, the specific products you choose, or your city’s current plan check queue.
Rental income, honestly
Two things worth being straight about. State law requires that an ADU rental be for a term longer than thirty days, so the short-term letting income some people have in mind is not available. And rent varies a great deal by city, by unit size, and by whether the unit has its own private entry and some outdoor space a studio garage conversion and a two-bedroom detached unit are different products.
The useful way to frame it: work out what you would need monthly to cover the financing, get a real local rent figure from a letting agent rather than a national average, and compare the two. If they are close, the unit substantially pays for itself over time. If they are not, the case has to rest on something else housing a family member, a home office, or the added property value.
How long the whole thing takes
| Phase | Typical duration |
|---|---|
| Feasibility and site assessment | 1 to 2 weeks |
| Design and engineering | 4 to 8 weeks, or considerably less on a pre-approved plan |
| Completeness determination | 15 days, statutory |
| Decision on a complete application | 60 days, statutory |
| Site work and foundation | 2 to 4 weeks |
| Framing through finish | 12 to 20 weeks |
| Final inspection and certificate of occupancy | 1 to 2 weeks |
When an ADU is the wrong answer
When the ceiling height in the garage cannot be made to work without cutting the slab. When you genuinely need the garage. When the unit would sit so far from the services that trenching erases the saving over a different route. When the only viable position destroys the usable garden and you have young children. And when what the household actually needs is more space in the main house, in which case an addition is the honest answer even though it has less tax and rental appeal.
Frequently asked questions
How much does an ADU cost in Contra Costa County?
It depends on the route far more than the size. A garage conversion starts with a slab, walls and a roof already paid for; a detached unit starts with nothing and adds trenching and site access. Our estimator gives a planning range in about thirty seconds based on type, size, finish level and the specific complications on your lot.
How long does an ADU permit take?
State law sets the clock. A city must determine completeness within fifteen days, and must approve or deny a complete application within sixty days. Concord states that pre-approved ADU plan permits are issued considerably faster, because only the site plan requires review.
What are pre-approved ADU plans?
Plan sets the city has already reviewed and approved, which you download and submit with your own site plan. Both Concord and Martinez run programmes. They carry expedited review and reduced permit and plan check fees but if you modify the plans, additional review and fees apply.
Can I convert my garage into an ADU?
Usually, if the ceiling height works, the slab is sound, the setbacks allow it and the services are reasonably close. Converted-space ADUs are generally treated more leniently than new construction. Measure from the slab to the lowest joist before anything else.
How big can an ADU be?
Up to 1,200 square feet under current state law, regardless of lot size. Local height and setback rules still apply, so confirm the specifics for your city.
Do I need to live on the property?
California has barred local owner-occupancy requirements for standard ADUs. Rules for junior ADUs and for rental terms are separate, and a rental must be for longer than thirty days.
Is it cheaper to build an ADU or an addition?
An addition adds space to the house you live in; an ADU adds a separate dwelling, and they serve different needs. Where either would meet the need and the calendar matters, the ADU has one real advantage: the permit clock is set by statute. No such certainty applies to an addition.
