An accessory dwelling unit is the most cost-effective square footage most Concord homeowners will ever add. It is also the project where the rules change most often, and where a bad assumption early costs real money later.
Here is the practical version of what a general contractor in Concord needs you to understand before you draw anything.
What Concord allows
California state law now does most of the heavy lifting. Cities, Concord included, are required to permit ADUs on nearly every residential lot, and local ordinances have been rewritten to match. In practice you have three paths.
Detached ADU — up to 1,200 square feet. A standalone structure in the rear or side yard. This is the version most people picture: a small house with its own entrance, kitchen, bathroom, and address. It carries the most freedom in design and the highest cost per square foot, because you’re building a complete structure from footing to roof.
Attached ADU. Built onto the existing house, sharing at least one wall. Size is generally capped as a proportion of the primary dwelling’s floor area, with a floor that guarantees you can always build at least 800 square feet regardless of that math. Attached units can be cheaper to service — utilities are closer — but they involve cutting into an existing structure, which brings its own surprises.
JADU — junior accessory dwelling unit, up to 500 square feet. Carved out of the existing footprint of the house, typically a converted bedroom or a portion of an attached garage. A JADU may share a bathroom with the main house and needs only an efficiency kitchen. It is the cheapest legal path to a rentable unit, and it is frequently overlooked.
Garage conversions deserve a specific note. Converting an existing detached garage is often the best value on the lot — the slab, walls, and roof already exist. But an old garage slab was never poured to house standards, and the framing was rarely built to carry insulation, drywall, and a code-compliant roof assembly. Budget for what’s underneath, not just what you can see.
Setbacks, height, and the rules that shape the box
Setbacks. State law guarantees a 4-foot side and rear setback for a new detached ADU. You cannot be forced further back than that. Front setbacks still follow the underlying zoning for the lot, which is why most detached units land in the rear yard.
Height. A detached ADU is generally allowed at least 16 feet, and 18 feet or more where the lot is near transit or where the unit is attached to a multifamily building. If you want a second story, confirm the exact allowance for your parcel before designing one.
Conversions of existing space get more latitude. If you’re converting an existing legal structure within its existing walls, setback and height requirements are generally satisfied by the structure that’s already there.
Parking. In most cases no replacement parking is required for an ADU, including when you convert a garage. This is one of the biggest practical changes of the last several years and it’s still news to a lot of homeowners.
Owner occupancy. The rule most likely to be misremembered. Owner-occupancy requirements for standard ADUs have been suspended under state law for years. JADUs are the exception: those still carry an owner-occupancy requirement, recorded as a deed restriction. If you’re planning to rent out both the house and the unit, that distinction decides which one you should build.
Short-term rentals. ADUs are intended as long-term housing. Rentals under 30 days are generally not permitted. Plan on a lease, not a listing.
The permit timeline
For a typical Concord ADU, expect three to six months from starting drawings to holding a permit.
- Design and drawings: 4–10 weeks. Site plan, floor plans, elevations, structural, and Title 24 energy compliance.
- Plan check: 60 days by statute. The city must approve or return comments on a complete ADU application within 60 days. “Complete” is doing a lot of work in that sentence — an incomplete submittal restarts the clock.
- Corrections and resubmittal: 2–6 weeks. Almost every set comes back with comments. This is normal, not a red flag.
- Utility and school-fee coordination: overlapping. Handled alongside the above rather than after.
Then construction: 4 to 7 months for a detached unit, less for a conversion. Plan the whole arc as a year, and be pleasantly surprised if it isn’t.
The three site conditions that change the budget most
Two identical 800-square-foot ADUs on two different Concord lots can differ by $80,000. Almost always it comes down to these three.
1. Slope. A flat pad is cheap. A sloped rear yard means engineered foundations — stepped footings, retaining, sometimes piers — plus drainage, plus grading, plus soil work and the report that justifies it. On a meaningfully sloped lot, foundation and site work alone can add $30,000–$60,000 before a single wall goes up.
2. Utility connections. The distance from your existing service to the new unit is a direct multiplier on trenching cost. Then the real question: does the existing electrical panel have capacity? A 100-amp panel serving a full house usually cannot also serve an ADU, and a panel upgrade plus a possible service upgrade from the utility adds both cost and weeks. Sewer matters too — if the new unit sits below the existing lateral, you’re looking at an ejector pump and the maintenance that comes with it.
3. The existing structure, on conversions. Once you open up an older garage, you find out what you actually bought. Slabs poured too thin or without a vapor barrier, framing that won’t take the new loads, foundations that don’t meet current requirements, and knob-and-tube or ungrounded wiring are all common in Concord’s older housing stock. A conversion budget without a contingency for this is not a budget.
How to start
The cheapest hour you’ll spend on this project is the first one — a walkthrough of the actual site with a licensed contractor, before any drawings exist.
What you want out of that visit: where the unit can physically go, where the utilities are and what the panel can carry, whether the lot slopes enough to change the foundation, and a realistic range for your specific parcel. That conversation reliably saves people from spending $8,000 on drawings for something the site was never going to support economically.
Pagac & Co Construction has been building in Concord since 2009, and we hold CA General Contractor License #812248. Site consultations are free, and you’ll get a written, transparent, line-item estimate rather than a number with no arithmetic behind it.