San Dimas Zoning and ADU Guide for Homeowners

Last updated November 13, 2026. Reviewed by the Cash Home Buyers CA team.

San Dimas publishes a 2025 Accessory Dwelling Unit requirements handout that caps ADU size at 1,000 square feet and requires 4-foot side and rear setbacks, and it directs homeowners to contact the Planning Division at planning@sandimasca.gov or 909.394.6250 before designing. This guide explains the figures the city handout gives, the utility and fire requirements it flags, the state law that sits underneath, and what to do if your home has an older garage conversion or unpermitted unit. The handout does not give every number, so confirm details with Planning.

San Dimas ADUs at a glance

Handout	Accessory Dwelling Unit Requirements (2025), City of San Dimas Building and Safety
Maximum ADU size	1,000 square feet
Setbacks	Minimum 4 feet from the sides and rear of the property
Sewer	ADU drain connects to the main unit’s building sewer; minimum 2% slope; no sewer line under a dwelling, structure, garage, breezeway, patio cover or porch
Electrical	Minimum 100A panel for the new residential unit
Required fixtures	Washer and dryer hookups
Fire	Plans must address Los Angeles County Fire Department fire flow and hydrant information
Planning contact	planning@sandimasca.gov or 909.394.6250

What the city handout says

The 2025 ADU requirements handout states that the maximum ADU size shall not exceed 1,000 square feet, that an ADU must include washer and dryer hookups, and that a minimum setback of 4 feet is required at the sides and rear of the property.

It says the sanitary drainage system for the new ADU shall be connected to the building sewer of the main unit and that new sanitary drainage must be separate and independent from any other building.

It adds a caution that the sanitary sewer line may not connect to, pass through or be installed under a dwelling unit, structure, garage, breezeway, patio cover, porch, steps or similar, with a minimum 2 percent slope.

It notes the minimum size panel for the new unit is 100 amps, and that plans must show service locations, meters and connections for water, gas and electrical.

The handout points to Los Angeles County Fire Department requirements for fire flow and hydrant location and size. It does not state height, parking, junior ADU, owner-occupancy or review-time rules, and it tells homeowners to contact the Planning Division before designing for additional requirements.

Other city resources

The city also posts a page on Assembly Bill 2234 covering pre-manufactured, detached ADUs with solar, junior ADUs and residential additions, and its fees and informational brochures page describes an ADU as an attached or detached residential dwelling unit that provides complete independent living facilities for at least one person.

The city’s housing element update, which sets out its housing policy for 2021 to 2029, also discusses ADUs. Ask Planning which documents apply to your lot, since a hillside, easement or overlay can change what is possible.

Zoning limits that apply to every project

Even without a building permit, zoning rules apply.

The planning FAQ says fencing may not exceed 42 inches in the front yard and six feet at the side and rear property lines, and that a fence over 6 feet or a storage shed over 120 square feet of floor area requires a building permit.

Garage conversions appear on the city’s permit list, and building without a permit appears on its code compliance list, so converting a garage without a permit can raise both issues at once. Our building permits guide and code enforcement guide explain each side.

State ADU law and San Dimas

California’s ADU statute, Government Code section 66314 and following, sets a floor that every city must follow, and local ordinances fill in the details.

The state requires ministerial review within 60 days of a complete application, bars a city from requiring more than a limited set of standards, and generally allows an ADU or conversion inside existing space without added setbacks.

The California Department of Housing and Community Development (HCD) publishes a handbook and keeps a list of the state requirements. Because state law changes often, treat the city’s current ordinance and handout as the final word on size, height, parking and fees for your lot.

ADUs, garage conversions and selling a San Dimas house

Many older homes have a converted garage or a backyard unit built without permits. Buyers and lenders ask about them, and California sellers generally disclose known unpermitted work. Our guide to selling as-is with unpermitted work explains the rules, and our Prop 13 guide explains how new construction can affect property tax. Before you list:

• Check city records for permits on any ADU, conversion or addition.
• Ask Planning whether the structure could qualify under current rules.
• Estimate plans, fees and corrections needed to legalize it.
• Compare that with selling as-is and letting a cash buyer take on the legalization.

Questions to ask Planning before you design

• Does my lot and zone allow an ADU, a junior ADU, or both?
• Is a pre-approved or pre-reviewed ADU plan available?
• What are the current impact, utility and school fees for the size I am considering?
• Can my existing garage or accessory structure be converted without changing its footprint?
• Are there fire, slope, easement or utility constraints on my lot?

Junior ADUs and garage conversions in more detail

State law treats a junior ADU as a unit of up to 500 square feet created inside the walls of an existing or proposed single-family home, with an efficiency kitchen and a recorded restriction that keeps it from being sold apart from the house.

A garage conversion is a different path: the unit replaces the garage, and state law generally does not require the city to demand replacement parking. Both are common sources of unpermitted space in older San Dimas neighborhoods, which is why buyers’ agents and appraisers ask for the permit record.

If a conversion was done without a permit, the usual fix is to submit as-built drawings, pay the fees and pass inspection, which can take months and cost real money.

Fees, utilities and property tax

State law limits the impact fees a city can charge on smaller ADUs and on conversions of existing space, and it limits when a separate utility connection can be required.

Larger new units can still carry impact fees, school fees and water or sewer connection charges, so ask for an itemized estimate before you commit to plans. New construction is generally reassessed for property tax under Prop 13, which means an ADU can raise the bill for the whole property. Our Prop 13 guide explains how reassessment works.

Why ADUs matter for resale

A permitted ADU can add rental income and appeal for some buyers, but an unpermitted one can complicate financing and appraisal. Lenders typically want permitted living area, and an appraiser may not count unpermitted space.

Because an ADU generally cannot be sold separately from the main home, its value comes through the sale of the whole property.

If you inherited a house with an old backyard unit, or bought one with a converted garage, check what the city has on file before you decide how to sell. Our probate home sale guide covers inherited property.

How to sell a house in San Dimas

1. Ask the city for the permit record on any ADU or conversion.
2. Collect plans, inspection records and any city notices.
3. Request a no-obligation cash offer for the house as it stands.
4. Choose your closing date.
5. Close with no repairs, legalization or showings required.

See also our San Dimas page, the San Dimas as-is sale page, the inherited house page, the San Dimas cash offer process and the Los Angeles County hub.

Frequently asked questions

What is the maximum ADU size in San Dimas?

The city’s 2025 handout says an ADU shall not exceed 1,000 square feet.

What setbacks apply to a San Dimas ADU?

A minimum 4-foot setback is required at the sides and rear of the property, per the city’s handout.

Who do I contact before designing an ADU in San Dimas?

The Planning Division at planning@sandimasca.gov or 909.394.6250.

Does the city handout say anything about ADU parking or height?

No. The handout does not state parking or height numbers, so ask Planning and check current state ADU law.

Can I convert my garage to an ADU in San Dimas?

Garage conversions need a building permit under the city’s permit list. Ask Planning whether your garage qualifies before you start.

Can I sell my San Dimas house with an unpermitted ADU or garage conversion?

Yes. You can disclose it and sell as-is, including to a cash buyer. Our statewide guide explains the disclosure rules.

More guides

• We Buy Houses in San Dimas
• We Buy Houses in Los Angeles County
• Los Angeles County Property Tax Guide
• San Dimas Building Permits Guide
• San Dimas Code Enforcement Guide
• Sell a San Dimas House As-Is
• San Dimas Cash Offer Process
• Glendora Building Permits Guide
• All California Guides

Sources

• City of San Dimas: Accessory Dwelling Unit Requirements (2025)
• City of San Dimas: Planning FAQ
• City of San Dimas: When Do I Need a Permit

This guide is general information from public sources, not legal advice. Confirm current rules, fees and hours with the city.