Reconfigure or extend the main home
Consider a better use of existing rooms before expanding. A ground-floor or upper-story addition introduces questions about foundations, structure, roof geometry, and connections to the household's living space.
ADUs & Home Additions in Contra Costa County.
Barcci / Residential design + build
More space starts with the right kind of space.
Discuss your project ↗More space starts with the right kind of space.
More space can solve very different problems. A home addition expands the main residence; an ADU creates independent living space on the same property. Start with who will use the space, the privacy they need, and the existing home's limitations. Then compare the options against the parcel, utilities, and applicable approvals before fixing the footprint.
Convert existing space
A garage or another suitable area may offer a different starting point from new construction. Existing floor levels, structure, moisture conditions, utilities, and intended use still need investigation.
Create an attached or detached ADU
Compare privacy, access, outdoor-space preservation, and utility routing. Independent living facilities make an ADU different from a simple backyard office; choose the use before choosing a plan.
Before a price, a scope.
Make the important decisions visible.
Use this checklist to prepare for the first conversation. It is a planning aid, not a permit checklist or an assertion that every item applies to your home.
- Property address and assessor parcel number
- Intended use and relationship to the main household
- Prior plans, permit records, and survey information
- Existing structures, easements, and site constraints
- Water, wastewater, power, and service-capacity information
- Construction access and outdoor space to preserve
- Applicable state/local pathway and association documents
- A complete proposal separating assumptions from confirmed work
Budget with context.
What changes the investment?
Compare proposals on included work, not just the bottom line. These are the cost drivers to discuss for your property.
- Conversion versus new construction
- Foundations, structural work, and excavation
- Retaining work and site access
- Utility routes, connections, and service capacity
- Kitchen, bathroom, roof, and window specifications
- Design, reports, applicable fees, and concealed conditions
No countywide price can replace a defined scope. Ask which items are confirmed, which are allowances, and which remain excluded.
Read our broader adus & home additions guide ↗The location changes the questions.
Contra Costa is a service area—not one permit office. Verify the parcel, scope, and current requirements with the authority that reviews the work.
Start with current state guidance
Use HCD's current ADU handbook alongside local review. An ordinary addition and an ADU can have different pathways; do not apply a generic hillside or association assumption to every ADU proposal.
California HCD — ADU handbook (PDF) ↗San Ramon: permit-ready is not site-approved
The City's permit-ready plan guidance still requires site-specific documentation, permits, outside-agency approvals, and inspections. Check a standard plan against the actual property before treating it as ready to build.
San Ramon — permit-ready ADU plans ↗Unincorporated communities: separate the approvals
For Blackhawk, Alamo, and Diablo, use the County's ADU process to establish planning and building requirements. The parcel and proposal determine the pathway, not the postal city alone.
Contra Costa County — ADUs ↗Planning guidance reviewed September 12, 2026. Codes, programs, and agency processes can change. These sources are not endorsements of Barcci Builders. Compare county, city, and town review paths ↗
See the work. Look closer.
Selected Barcci projects from our Bay Area portfolio. These are real project photographs with their actual locations—not representations of completed Contra Costa work.
ADUs & Home Additions: questions before you start
Clear starting points for the decisions ahead. Your address and scope determine the final requirements.
Should I build an ADU or enlarge my house?
Choose based on the intended use. An addition generally serves the main household's connected space; an ADU provides independent living facilities. Compare both against the parcel, privacy needs, utilities, and desired outcome.
Do permit-ready ADU plans eliminate project-specific approvals?
No. San Ramon's guidance retains site-specific review, permits, outside-agency approvals, and inspections. A standard plan alone does not establish whether its footprint or utility assumptions suit your property.
Should I assume an HOA prevents an ADU?
No. Review the actual documents and the current HCD guidance rather than assuming an automatic prohibition. State rules and reasonable property-specific restrictions need to be considered alongside the public approval pathway; association review does not replace permits.
What affects the cost of adus & home additions in Contra Costa County?
A proposal needs the property and defined scope. Important cost drivers include conversion versus new construction; foundations, structural work, and excavation; retaining work and site access; utility routes, connections, and service capacity; kitchen, bathroom, roof, and window specifications; design, reports, applicable fees, and concealed conditions. We separate assumptions, allowances, and exclusions rather than treating a countywide figure as a quote.
When can construction start?
First establish design readiness, applicable approvals, material availability, site access, and the agreed construction scope. Agency review and existing conditions can affect the sequence. Discuss your target date with us; a website guide is not a promised start date.
Where is Barcci Builders based, and how can I review its work?
Barcci Builders is headquartered in Los Gatos and serves the listed Contra Costa communities. Its project galleries identify the actual locations of completed Bay Area work. This service guide is not a claim of a Contra Costa branch office or a local project record.
Find your community
Explore the design considerations and local review paths for your part of Contra Costa County.
Unincorporated community
Blackhawk ↗
A more personal home, considered down to the last detail.
City / townDanville ↗
Make the home you love work beautifully for the life you live.
City / townSan Ramon ↗
An addition should feel like it always belonged.
City / townWalnut Creek ↗
More light, better flow, and details that make everyday life easier.
Unincorporated communityAlamo ↗
Reimagine the layout. Respect the setting.
City / townLafayette ↗
A considered transformation, from the kitchen to the hillside.
City / townOrinda ↗
Keep what makes the setting special. Rethink how the home lives.
City / townMoraga ↗
Room to gather. Space to retreat. A home that works as one.
Unincorporated communityDiablo ↗
Preserve the character. Make room for what comes next.
Start with a conversation.
Let’s talk about your Contra Costa home.
Tell us the property’s community and what you want to change so we can discuss the right scope and next steps.
Los Gatos headquarters · California Contractor License #1086047. Verify the current license record ↗