ADUs & garage conversions · Walnut Creek · Lafayette · Danville · Orinda

ADU Builders in Walnut Creek, CA

In Contra Costa, garage-conversion ADUs typically run about $150,000–$250,000+ and new detached ADUs about $250,000–$450,000+ (Mazzamuto Construction, 2026). State law requires cities to approve qualifying ADUs within 60 days.

Get ADU Pricing Call (925) 709-5438

Typical backyard ADU placement: 4 ft from the side and rear lines, sewer tied in downstream of the main cleanout.

What are you building?

Pick the ADU that fits your lot

Detached ADUA separate backyard cottage, up to 1,200 sq ft. Most privacy, highest cost.$250K–$450K+ → Garage conversionTurn an existing garage into a legal unit. No new setback, no replacement parking.$150K–$250K+ → Prefab / modularFactory-built unit set on a local foundation. Compare all-in, not base, prices.From $298,800 all-in → Not sure yetDetached vs attached vs garage vs JADU vs prefab, side by side.Compare types →

Cost

What an ADU costs in Contra Costa

Most Walnut Creek-area ADUs land between about $150,000 and $450,000+ all-in. The type sets the baseline; your lot sets the rest: slope, access, sewer route, electrical panel, trees and finish level. Real quotes can come in below or above these ranges.

Full cost guide with 500 and 800 sq ft examples →

Junior ADU (JADU)Mazzamuto Construction

$110,000–$180,000+

Garage conversionMazzamuto Construction

$150,000–$250,000+

Attached ADUMazzamuto Construction

$220,000–$380,000+

Detached ADUMazzamuto Construction

$250,000–$450,000+

Prefab, 800 sq ft, all-inIMKAT (Bay Area)

$300,000–$450,000+

Contractor-published ranges for Contra Costa and the Bay Area, 2026. Prefab: IMKAT; Abodu lists Walnut Creek all-in pricing from $298,800 (Abodu).

State law

What California guarantees on a single-family lot

60 days. A city must approve or deny a complete ADU application within 60 days, or it is deemed approved. Gov. Code §66317
4 ft setbacks. A city can't require more than 4 ft from side and rear lines for a new ADU, and none for converting existing space. §66314
800 sq ft, 16 ft high. Cities must allow at least an 800 sq ft ADU and a 16 ft detached height; up to 1,200 sq ft detached is possible. §66321
No impact fees under 750 sq ft. Larger units pay proportionally; conversions of existing space need no new utility connection fee. §66324

All rules, city plans and utilities →

The 4 ft limit applies to new ADUs; converting a garage keeps its existing footprint.

City programs

Plans and rebates at a glance as of October 2026

Where	Pre-approved plans	Rebates / fee relief	Source
Walnut Creek	City pre-approved and third-party designer plans	SD7 rebate share fully claimed July 15, 2025; backup list only	City plans · Rebate
Lafayette	County PADU vendor plans; city library in progress	Impact fees for new ADUs cut 50% through 2026; SD7 rebate up to $15,000	City ADU page
Danville	Garden Cottage: detached plans, no charge to download	Lower permit fees with those plans; SD7 rebate up to $15,000	Town page
Orinda, Moraga, Clayton	County PADU vendor plans	SD7 rebate up to $15,000 (city allocations vary)	County PADU
Alamo, Blackhawk, Diablo	County PADU vendor plans	Not in the SD7 program (unincorporated)	SD7 page

SD7 = the state-funded ADU Accelerator for Senate District 7, administered by the Town of Danville. Funds are limited and allocated city by city; confirm status with your city before counting on a rebate. Rebates & financing →

Process

From feasibility to move-in

FeasibilitySetbacks, slope, trees, fire zone, HOA, sewer route and panel capacity decide what fits.
DesignCustom drawings or a pre-approved plan, plus structural and Title 24 energy work.
PermitCity or County review; a 60-day decision once the application is complete. Central San and your water provider sign off on connections.
BuildFoundation, framing, utilities and inspections through final.

Typical schedule for a site-built detached ADU. Garage conversions and prefab installs often run shorter; redesigns and hillside work run longer.

Areas

Walnut Creek and the Lamorinda–San Ramon Valley corridor

Each city runs its own ADU program. Walnut Creek handles its own permits; Lafayette, Orinda and Moraga use County building services and the County's pre-approved vendor plans; Danville offers its own Garden Cottage plans.

Lafayette — ministerial approval, 50% impact-fee cut through 2026
Danville — Garden Cottage plans, SD7 rebate administrator
Orinda — steep lots, fire-zone construction, County plans
All areas — Moraga, Alamo, Pleasant Hill, Concord, Clayton

Check any builder

Before you sign: CSLB basics

The Contractors State License Board says a B – General Building contractor can build an ADU. Because an ADU counts as home improvement, you need a written home improvement contract, and the down payment can't exceed $1,000. Never let payments get ahead of the work (CSLB FastFacts: ADU).

Look up a license · How to compare ADU builders →

Bid checklist

Design, engineering and Title 24 included?
Who pulls permits and pays plan-check fees?
Sewer, water and electrical connection fees
Site work, grading and hillside allowances
Finish allowances in dollars
Schedule, payment milestones, warranty

FAQ

ADU questions Walnut Creek homeowners ask

How much does a 500 sq ft ADU cost in California?

In the Bay Area, new detached ADUs run about $450–$700+ per square foot (IMKAT, 2026), which puts a 500 sq ft unit around $225,000–$350,000+ before unusual site work. Small units cost more per foot because the kitchen and bath are spread over fewer square feet. A garage conversion of similar size usually costs less. See the ADU cost guide.

What are the drawbacks of an ADU?

Cost and time are the big ones: six figures and often a year or more from design to move-in. You also give up yard or garage space, live through construction, may need a panel or sewer upgrade, and the new construction adds to your property tax. An ADU can be rented but not sold separately from the house.

Is there a $40K ADU grant?

Not right now. The state's CalHFA ADU Grant (up to $40,000 for pre-development costs) says its latest round of funding has been fully allocated. Locally, Walnut Creek's share of the Senate District 7 ADU Accelerator rebate (up to $15,000) was fully claimed as of July 15, 2025. Details on the rebates page.

How long does it take to build an ADU in Walnut Creek?

Plan on roughly a year for a site-built detached ADU: about 1–3 months of design, a permit decision within 60 days of a complete application under state law, and 5–8 months of construction. Garage conversions and prefab units often finish sooner; hillside lots and redesigns add time.

Do I have to live on the property to rent out an ADU?

No. State law bars cities from requiring owner occupancy for an ADU (Gov. Code §66315). Cities may require rentals of more than 30 days. A junior ADU is different: owner occupancy can apply when it shares sanitation facilities with the main home.

Does an ADU builder need a license?

Yes. The CSLB says a B (General Building) contractor can build an ADU, a written home improvement contract is required, and the down payment cannot exceed $1,000. Check any license at cslb.ca.gov before you sign.

Next step

Ready to price your ADU?

Share your lot and goals. A local ADU builder can review feasibility and give you a quote.

Request a Quote Call (925) 709-5438