ADU Builds in California, Cost Drivers, Timeline, Permit Steps, and a Site Checklist
- Kevin Morrison
- Dec 14
- 4 min read
An ADU, also called an accessory dwelling unit, adds a second living space on one property. Homeowners build ADUs for family housing, rental income, or long term flexibility. This guide covers the main ADU types, a quick site screen, the ADU permit process, a realistic ADU timeline, and the cost drivers that move the budget. San Gorgonio Builders serves Banning,
Beaumont, Cherry Valley, and nearby areas within about a two hour drive. San Gorgonio Builders is a California licensed General B contractor. Call (951) 972-9509 to schedule an ADU consult.
Start with the right ADU type
Choose the ADU type based on yard space, privacy, access, and existing structures. A detached ADU gives separation and flexible layout options. An attached ADU shares a wall with the main home and often shortens utility runs. A garage conversion ADU uses an existing structure, which often reduces exterior work, but the project still needs code upgrades in many cases. An above garage ADU keeps yard space open, but stairs and structural work add scope. Start with your goal, family space, rental income, or future downsizing, then match the goal to the type.
Detached ADU, best privacy, more site work
Attached ADU, shorter utility runs, careful layout at shared wall
Garage conversion ADU, shorter build phase on many lots, heavy upgrades sometimes
Above garage ADU, keeps yard open, higher structural scope
Junior ADU, smaller footprint inside a primary home in many cities
A site screen prevents redesign and plan check delays. Walk the property and confirm access for materials, trades, and inspections. Locate utilities early because utility upgrades drive both cost and schedule. Identify the main electrical panel, overhead service lines, water meter, gas meter, sewer cleanout, and any easements. Check slope and drainage so the foundation and flatwork do not create water issues. Note hardscape, patios, pool equipment, and tree roots that affect excavation. Take photos of the side yards, panel, meters, driveway, and the proposed ADU location. Bring measurements and photos to your contractor meeting, then choose a concept layout.
Measure side yard widths and rear yard space
Confirm driveway width and access path to the build area
Locate main electrical panel and note available breaker space
Confirm panel amperage and distance from panel to ADU
Locate water meter and route options for a new water line
Locate gas meter and route options if the ADU uses gas
Locate sewer cleanout and route options for the sewer connection
Check slab and framing conditions for a garage conversion ADU
Note slope, drainage paths, and retaining walls
Note trees and root zones near the proposed foundation
List parking layout for the property
Check HOA rules if the property has an HOA
ADU permit process in plain terms
Cities review ADU plans through a ministerial path in many cases, meaning the city checks for objective standards like setbacks, height, and building code compliance. Your plan set matters. A complete plan set reduces correction cycles. Most ADU permit issues come from missing site information, unclear utility routing, incomplete structural details, and incomplete energy documentation. Plan review moves faster when the site plan is accurate and utility runs are clear. Respond to corrections fast, keep revisions organized, and avoid design changes during review.
ADU timeline phases
Most ADU projects follow the same phases. Phase 1, site screen and concept layout. Phase 2, design and engineering, then city submittal. Phase 3, plan review and corrections, then permits. Phase 4, site prep and foundation. Phase 5, framing and rough plumbing, electrical, and HVAC. Phase 6, insulation, drywall, and interior finishes. Phase 7, exterior tie ins and final inspections. Utility work often drives delays, especially an electrical panel upgrade, long sewer runs, or gas sizing changes.
ADU cost drivers that move budgets
ADU cost changes most from site conditions and utility scope. Long trench runs add labor and restoration costs. A panel upgrade adds parts, labor, and coordination. Sewer depth and route matter, especially when the run crosses concrete or ties in deep. Foundation type changes cost based on soil and slope. Finish level changes cost through cabinets, countertops, tile, fixtures, windows, and doors. Set finish level early and keep a contingency line for unknown site items during excavation.
Design and engineering
Permits and fees
Site work and grading
Utilities and service upgrades
Foundation and framing
Roofing and exterior finishes
Windows and doors
Plumbing, electrical, HVAC
Insulation and drywall
Cabinets, countertops, tile, flooring
Fixtures and appliances
Paint, trim, hardware
Final clean and punch list
ADU FAQ
What ADU type often moves fastest
A garage conversion ADU often moves faster when slab, framing, and setbacks work, and utility tie ins stay short.
What items move ADU cost the most
Utility runs, an electrical panel upgrade, sewer depth, grading, and finish selections.
What should happen before paying for full plans
Start with a site screen, basic measurements, utility locations, and a concept layout that fits setbacks and access.
Call for an ADU consult
If you want an ADU budget range and a clear scope, start with a site screen and a written plan for utilities, access, and schedule. San Gorgonio Builders serves Banning, Beaumont, Cherry Valley, and nearby areas within about a two hour drive. Call (951) 972-9509 to schedule an ADU consult.
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