ADUs & Additions in Diamond Bar

City of Diamond Bar Community Development Department, Building and Safety Division. Building permits come from the Building and Safety Division of the Community Development Department at 21810 Copley Drive, while grading is permitted separately through the Public Works/Engineering Department, whose grading plan check calls for a complete soils and geology report. The Development Code treats a parcel with grades over ten percent as hillside, and Chapter 22.22 sends hillside development through development review, measures building setbacks from the edge of the building pad and caps retaining walls associated with lot pads at four feet, with terraced walls considered lot by lot only where unusual or extreme conditions call for more. On single-family lots the code sets a 5-foot side and rear setback for a pool or spa and 42 inches for pool and spa equipment. The State Fire Marshal's March 24, 2025 map for the city shows most of the southern hills as a Very High Fire Hazard Severity Zone, where Chapter 7A construction and defensible space requirements may apply, and the city quotes a 15-working-day ADU plan check once Building and Safety accepts the plans, with 10 working days for each resubmittal. We prepare the drawings, make the submittal and handle plan-check corrections ourselves, so you are not the one chasing plan check.

Many homes sit on graded pads above or below privately owned hillsides, and the Development Code measures the rear setback, 20 feet in the RL and RLM districts, from the property line or from the buildable pad on a descending slope, whichever applies. For new subdivisions it sets minimum lots of one acre in the RR (rural residential) district, which the code reserves for hillside areas intended for rural living, 10,000 square feet in RL and 8,000 square feet in RLM, and the guard-gated Country Estates holds 931 custom homes and lots on hillside terrain. One local condition that shapes the work: The Housing Element describes rolling topography and local soils that create numerous potential landslide areas, and while many historical slides have been stabilized, some potential landslide areas remain in the eastern portion of the city; the city's grading permit application notes that geological or engineering reports may be required in hillside areas.

All of it, including The Country Estates, Diamond Ridge Estates, Palomino and Sunset Crossing. Our crews run every job out of our Cahuenga Pass base in Los Angeles, with our own people on site.

Detached ADU: generally $150,000 to $400,000+ across Southern California. What moves it is square footage, finish level, utility runs, and site access; garage conversions typically run well below a ground-up detached build. We give line-item pricing after seeing the site rather than quoting a number over the phone.

Plan a realistic year. Design and engineering typically take one to three months, city plan check and corrections add several more, and construction usually runs four to eight months depending on size and site access.

Usually yes, and often substantially, because the foundation, walls and roof already exist. The trade-offs are that you lose the garage, and older slabs and framing sometimes need upgrading to meet current code.

California ADU law is designed to allow long-term rental, and an ADU is one of the few ways to add rentable square footage to a single-family lot. Short-term rental rules vary by city, so confirm your local ordinance before counting on that income.

Yes. Plans, engineering, Title 24 and the city submittal are all handled in-house, including responding to plan-check corrections. That is a service we sell on its own, so it is not something we hand off.