Building an ADU in Concord? Here Are the Three Numbers That Decide

A neighbour finishes a small cottage at the back of their lot and suddenly everyone on the street is doing the arithmetic. Could the same thing fit behind your house? In Concord the answer usually turns on three numbers — how big, how far from the fence, and whether you have to give up a parking space — and the city has written all three down.

Concord allows a second unit up to 1,000 square feet on most lots, or 1,200 square feet on lots of 12,000 square feet or more, with four-foot side and rear setbacks and a detached height limit of 16 feet (§ 18.200.180). A complete building permit application is processed ministerially within 60 days — no hearing, no discretion.

Key numbers

What Concord is actually regulating

An accessory dwelling unit is a second, self-contained home on the same lot as the main house, with its own permanent provision for living, sleeping, eating, cooking and sanitation. A junior accessory dwelling unit is a smaller version — up to 500 square feet — carved out entirely inside the walls of an existing or proposed single-family house or its attached garage, and it must include an efficiency kitchen. Concord's own definitions govern this section and prevail over conflicting definitions elsewhere in the municipal code (§ 18.200.180); the general zoning glossary sits in § 18.20.020.

The city says outright why the section exists: to comply with state law that lets local jurisdictions set their own standards while increasing the supply of smaller, more affordable housing.

Size, and the floor that overrides the cap

The living area must be at least 150 square feet. The ceiling is the lesser of half the floor area of the principal dwelling, or a figure set by lot size: 1,000 square feet on lots under 12,000 square feet, 1,200 square feet on lots of 12,000 square feet or more. Attached garages or carports are excluded from that count up to 230 and 460 square feet respectively.

Then comes the sentence that rescues small lots: that limitation is waived if it would hold the unit below 800 square feet. So a modest house on a modest lot does not end up with a 400-square-foot cap — the half-the-house rule cannot push you under 800.

Setbacks and height

Interior side and rear setbacks are at least four feet. Front and corner-side setbacks follow whatever the principal dwelling has to meet — and where the house already encroaches into its setback, the new unit may encroach to the same extent.

Height splits by type. An attached unit may not exceed 25 feet, or the height limit that applies to the main house, whichever is lower, and never more than two storeys. A detached unit is held to 16 feet, rising to 18 feet where the lot is within half a mile walking distance of a major transit stop or high-quality transit corridor, with two more feet allowed to match the roof pitch of the main house. A single-storey unit may also exceed 16 feet where the planning division finds the extra height is needed to sit better with the roofline or architectural style of the house. Converting an existing detached accessory structure carries no height limit at all.

Parking — the number that catches people out

This is where Concord differs from some of its neighbours:

one offstreet parking space is required per accessory dwelling unit or per bedroom, whichever is less

— § 18.200.180

That space is in addition to the parking already required for the main house. It may be uncovered, tandem, or in the front setback if it sits within an existing or proposed driveway, and the driveway may be widened for it — as long as the result does not pave more than half the required front setback or yard.

Five situations remove the requirement entirely: the unit is within half a mile walking distance of public transit; it is in a designated historic district; it is part of the house or a conversion of an existing accessory structure; on-street parking permits are required but not offered to the occupant; or a car-share vehicle site is within one block.

The exempt route

Separately from the standards above, the code recognises an exempt accessory dwelling unit — up to 800 square feet with four-foot side and rear yards — which is not subject to lot coverage, floor area ratio, open space and other development standards where applying them would prevent it being built. Limits based on a percentage of the main house, lot coverage, floor area ratio, front setback and open space must give way for a unit of that size that otherwise complies.

That mirrors a state guarantee: cities must ministerially approve certain combinations of units regardless of local standards (§ 66323), and state law separately bars a city from imposing parking standards in a list of situations (§ 66322). Read those in the right direction: they are limits on what Concord may demand, not requirements aimed at you.

Design, and the free plan sets

Colours and materials must be consistent with the main house using at least two of three approaches — one matching exterior colour, two matching exterior materials, or a matching roof slope. Miss two of the three and the project goes to discretionary design and site review instead.

An attached unit needs its own exterior entrance, not on the same side or facing the same direction as the main entrance, unless the house, topography, fencing or landscaping already screens it from every street view. Upper-storey windows should be staggered to avoid a direct line of sight into neighbouring homes.

Worth knowing before you pay for drawings: the city maintains preapproved plan sets — permit-ready architectural and structural packages for detached units in several types and sizes, provided free to the public. Preapproved units are exempt from the design standards above.

The rules that are not about the building

• Short-term rental is prohibited. The unit may not be rented for a term of under 30 days.
• You cannot sell it separately, though you may lease it separately. No subdivision of land or air rights may put the unit on its own lot. A narrow exception exists for units developed by a qualified nonprofit under state law (§ 66341).
• It does not count against density. A conforming unit is not treated as exceeding the allowable density for the lot — the same rule state law applies statewide (§ 66319).
• Existing problems on the property are not grounds for denial. Concord will not refuse a permit over nonconforming zoning conditions, building code violations or unpermitted structures that do not threaten health and safety and are unaffected by the work.

Units are allowed in the residential districts and in several commercial and downtown districts, including North Todos Santos, Community Office, Commercial Mixed-Use, Neighborhood Commercial, Downtown Pedestrian and Downtown Mixed-Use.

Cities amend these sections often, so confirm the current text before you commit to a design. Our Concord ADU guide sets the same rules beside the state baseline, and the Concord overview links to the rest of the city's code.

Sources

• Concord Development Code § 18.200.180 — Accessory dwelling units
• Concord Development Code § 18.20.020 — General terms
• California Government Code § 66319 — Density and accessory use
• California Government Code § 66322 — Parking standards
• California Government Code § 66323 — Ministerial approval
• California Government Code § 66341 — Separate sale to a qualified buyer