ADU Constructionin El Cajon, CA
ADU construction in El Cajon runs on state ADU law, and a state review letter found the City's own ordinance out of step with it in ten respects. The City's code allows one ADU and one junior ADU on a single-family lot and needs a building permit. Treat City numbers as ones to confirm with Planning.
El Cajon has plenty of room for ADUs on paper. Single-family detached homes are 44.9 percent of its 33,340 units, 14,955 houses, and many of them sit on lots laid out in the 1950s to 1970s with a garage and a back yard. About 55.5 percent of households rent, which is the market a second unit would serve.
This page starts with state law, then covers the size and height numbers, hillsides and fire, garage conversions, fees and the order of the job. The City's code text comes from a third-party mirror with machine-written summaries, because the official code site refused our tools, and the City has amended its ADU section several times since the state reviewed it. Confirm every City number with Planning. Price ranges and phases are on our ADU construction page.
El Cajon Planning and Building rules that apply to ADU construction
Where adu construction permits come from in El Cajon: City of El Cajon Community Development Department, Building and Fire Safety Division.
- Section 17.140.180 requires a building permit for any ADU or JADU. On a single-family lot the code allows one ADU and one JADU; an ADU is limited to 1,200 square feet of habitable space and never larger than the primary home, and a JADU to 500 square feet. A detached ADU needs at least 3 foot side and rear setbacks and may be 20 feet tall at a 5 foot setback, 16 feet at 4 feet, or 12 feet at 3 feet. ADUs under 750 square feet pay no impact fees.
- Parking for an ADU is one extra paved space, but none is required within one-half mile of public transit, when the ADU is part of the primary dwelling or an accessory structure, for a JADU, or within one block of car-share, and replacement parking is not required when a garage or carport is demolished for an ADU. Section 17.140.180(F) adds that for a JADU the owner must occupy either unit when they share sanitation facilities, a notice of restriction is recorded before final building permit approval, a JADU cannot be sold separately and the minimum tenancy is 30 days.
- California HCD's February 6, 2025 review letter found that El Cajon's ADU ordinance (No. 5142, adopted May 28, 2024) fails to comply with state ADU law in ten numbered respects, including the one ADU plus one JADU limit, the 1,200 square foot and primary-dwelling size caps, the parking exceptions, the sprinkler language, the attached ADU height rule and the JADU rule on attached garages. The City has amended section 17.140.180 since by Ordinances 5148, 5151 and 5173, so state law controls where the two conflict and the current text should be confirmed with Planning.
Sources for these rules
What law governs an ADU in El Cajon: state law, Ordinance 5142 or the City's later amendments?
State law leads. California's ADU statute, Government Code sections 66310 to 66342, says an ADU application is approved ministerially, without a hearing, and that local rules may use only objective standards. A February 6, 2025 letter from the state's Department of Housing and Community Development found that El Cajon's ordinance, No. 5142 of May 28, 2024, fails to comply with state law in ten numbered respects. The letter is on the HCD website.
The findings reach the one ADU plus one JADU limit, the 1,200 square foot and primary-dwelling size caps, the parking exceptions, the sprinkler language, the attached ADU height rule and the JADU rule on attached garages. The City has amended its ADU section since, through Ordinances 5148, 5151 and 5173, and we do not know what its reply to the letter said.
So the working rule is this. Where the City's number and the state's number disagree, state law controls, and the figures on our ADU page come from state law. Ask Planning which City standards it applies to your lot, and ask for the answer in writing.
How big can an ADU be in El Cajon, and how tall?
The City's code on the mirror, section 17.140.180, allows one ADU and one JADU on a single-family lot. It caps an ADU at 1,200 square feet of habitable space and never larger than the primary home, and a JADU at 500 square feet. A detached ADU needs 3 foot side and rear setbacks and may reach 20 feet at a 5 foot setback, 16 feet at 4 feet or 12 feet at 3 feet.
State law sets a floor under those caps. A city may not set a maximum below 850 square feet, or 1,000 square feet with more than one bedroom, and it must still allow an 800 square foot detached ADU with 4 foot side and rear setbacks. Cities must also allow at least 16 feet of height for a detached unit. That is why an ADU can fit on a lot where a house addition would not.
Compare that with ordinary single-family zoning. In the RS-6 zone, which uses a 6,000 square foot minimum lot, the mirror lists a 20 foot front setback, a 15 foot rear setback, a 35 foot height limit and 40 percent lot coverage. Accessory buildings in those zones need 6 feet between structures and cannot sit in an exterior side setback.
Hillside, wildfire and fire access rules for an El Cajon ADU lot
Parts of El Cajon are steep. Where the H Hillside Overlay applies, chapter 17.170 on the mirror limits cut and fill slopes to 2:1 and 30 feet, requires fill compaction to 90 percent and calls for a geotechnical report from a certified engineering geologist or licensed geotechnical engineer, including for retaining walls in areas of known geological hazard. Excavations of 50 cubic yards or less that are under a foot deep are exempt, which a utility trench may be and a pad cut will not be.
Wildfire is a City-level question. Ordinance 5167 of October 28, 2025 adopted a City map dated March 25, 2025 of Very High Fire Hazard Severity Zones and Wildland Urban Interface areas, kept at the Fire Department Administrative Office and the Building and Fire Safety Division, and Ordinance 5166 applies the 2025 Wildland-Urban Interface Code to the Very High parcels. One builder's site says the map has other tiers too, and the ordinance text we read names only Very High and WUI areas. Ask the City which applies to your parcel.
Fire service comes from Heartland Fire & Rescue, a joint agency of El Cajon, La Mesa and Lemon Grove described on the Lemon Grove fire page. Section 15.56.070 requires fire apparatus access roads at least 20 feet wide, or 16 feet for a single-family driveway up to 150 feet long serving no more than two dwellings, so a long flag-lot driveway needs a check before a unit is drawn at the back.
Garage conversion ADU in El Cajon and the parking that comes with it
A garage with a kitchen and a bathroom becomes a dwelling, and it is judged as one. State law protects a conversion of an existing garage into an ADU, with a JADU allowed alongside, and lets an owner add up to 150 square feet only for entry and exit. It also says replacement parking is not required when a garage is converted, and the mirror's City text says the same for a garage or carport demolished for an ADU.
The City's own parking rule asks for one extra paved space for an ADU, but none within one-half mile of public transit, when the ADU sits inside the house or an accessory structure, for a JADU or within one block of car share. Because those exceptions line up with the state's, most conversions need no new space, but ask Planning for the answer on your block.
One separate local rule matters. The mirror says an enclosed two-car garage in the RS zones must keep 20 by 20 feet of clear interior space, so a conversion removes a protected parking area the City otherwise cares about. Our guide to garage conversion permit steps shows the sequence of the work in Los Angeles, a different City with its own bulletin.
ADU fees, utility connections and renting the unit in El Cajon
Under state law an ADU of 750 square feet or less pays no impact fee, a JADU of 500 or less pays none, and larger ADUs pay in proportion to the main house. The City's code, as the mirror summarizes it, says the same for units under 750 square feet. Utility connection charges are separate from that exemption, and an ADU converted from existing space cannot be forced to get a new utility connection. School fees follow their own rules and may reach units over 500 square feet.
Renting is the common reason to build. State law lets a city require terms of 30 days or longer and bars owner-occupancy rules for an ADU. For a JADU, the City code asks for owner occupancy in the main house or the JADU when they share sanitation facilities, a recorded notice of restriction before final approval and a 30 day minimum tenancy, and it says a JADU cannot be sold separately.
Two items are worth asking about, and we could not confirm either. A search result describes a City ADU loan program, a deferred construction loan with a five year affordability term, on a City page we did not open. Some builders also say El Cajon asks for a video inspection of the sewer lateral. Ask Planning and Engineering whether either applies to you. No permit fee amounts are printed here because none were in the sources we read.
How long does it take to build an ADU in El Cajon, from application to final?
The state clock is the only published review time. Since January 1, 2026 an agency has 15 business days to say whether an application is complete and 60 days after that to approve or deny it when a house already stands on the lot, and silence past 60 days counts as approval. We found no El Cajon figure for how long review takes in practice, so we do not promise one.
The City's section 17.140.180 requires a building permit for any ADU or JADU, with plumbing, electrical and mechanical work inspected. Electrical permits lapse if work does not begin within 180 days or stops for 180 days. Floor plans, a site plan, utility routing and the foundation come first, and a new detached unit is commonly a slab with a framed shell.
As MDG Builders, CSLB #1040337, we draw the unit, run the permits and manage the schedule, while licensed foundation, framing, plumbing and electrical firms do the field work. Ask each for its license number and certificate of insurance. A unit built without permits is a poor foundation for a rental, and our ADU versus JADU guide compares the two unit types.
ADU photos from El Cajon and nearby homes
ADU scenes tied to El Cajon and the communities around it, from garage conversions and slabs to finished studios. Tap a photo to enlarge it.
What adu construction costs around El Cajon
A garage conversion to a studio or one bedroom runs roughly $100,000 to $200,000 on our ADU construction page, against $200,000 to $400,000 for a new detached unit of 400 to 800 square feet. Three independent sources set those ranges. A UC Berkeley survey of Los Angeles County projects from 2018 and 2019 found a $100,000 median, with garage ADUs at $90,000 and detached units at $180,000. The Terner Center's 2020 estimate for Los Angeles was $148,000, and Remodeling magazine's 2025 job cost of $178,536 recouped 39.9 percent at resale. None is an El Cajon quote, and San Diego County costs may differ. Permit and utility fees appear as their own lines in a written proposal. The ADU cost guide covers the details, and the room addition versus ADU guide helps choose between them. Other cities apply the same state law in their own way, so compare ADU construction in Dana Point and Los Angeles. A bathroom inside the unit is on our El Cajon bathroom page, a bigger attached addition on the El Cajon room addition page, and the El Cajon area page lists every trade we cover in the city.
| What it covers | Typical range |
|---|---|
| Garage conversion to a studio or one bedroom | $100,000 to $200,000 |
| New detached ADU, 400 to 800 sq ft | $200,000 to $400,000 |
| UC Berkeley survey, Los Angeles County median, 2018 and 2019 projects | $100,000 overall. $90,000 garage. $180,000 detached. |
| Terner Center and Berkeley CCI, 2020 | $148,000 Los Angeles estimate. $167,000 statewide average. |
| Remodeling Cost vs. Value 2025, Los Angeles | $178,536 job cost |
| Electrical and plumbing, Angi Los Angeles, January 2026 | Electrical $2,600 to $7,800. plumbing $3,900 to $10,400. |
| City permit and plan check fees | Not priced here |
El Cajon ADU questions, answered
How to build an ADU in El Cajon?
Start with state ADU law and a call to Planning, then draw plans, apply for a building permit and build under inspection. The City's code requires a building permit for any ADU or JADU. A state letter found the ordinance out of step with state law in ten ways, so confirm which City numbers Planning applies to your lot.
What is the maximum size ADU I can build?
The City's code, as the mirror shows it, caps an ADU at 1,200 square feet and never larger than the primary home, and a JADU at 500. State law sets a floor: cities must allow 800 square feet with 4 foot setbacks and cannot cap below 850, or 1,000 with more than one bedroom. Confirm the current City text.
How much does it cost to build an ADU in California?
By our sources, expect roughly $100,000 to $200,000 for a garage conversion and $200,000 to $400,000 for a new detached unit of 400 to 800 square feet. Those come from Los Angeles County surveys and a 2025 job cost, none for El Cajon, so a written proposal after a site visit sets the actual figure.
Do I need to provide parking for my ADU?
Often not. The City's code asks for one extra space but waives it within one-half mile of public transit, when the ADU is inside the house or an accessory structure, for a JADU and within one block of car share. State law adds that no replacement parking is needed for a converted garage. Ask Planning about your block.
What is the difference between an ADU and a JADU?
An ADU is a complete, independent unit with its own sleeping, cooking and sanitation, attached or detached, while a junior ADU is a unit of up to 500 square feet carved out of the existing house. El Cajon's code allows one of each on a single-family lot. Our ADU versus JADU guide compares the rules.
Can I rent out my ADU or sell it separately?
An ADU can be rented, and state law lets a city require terms of 30 days or longer without any owner-occupancy rule for an ADU. A JADU follows the City's owner-occupancy rule when sanitation is shared. An ADU generally cannot be sold separately. Short-term rental rules are separate, and we did not read El Cajon's.
How long does it take to get an ADU permit?
State law gives the City 15 business days to call an application complete and 60 days after that to approve or deny a complete one when a house stands on the lot, with silence counting as approval. We found no published El Cajon review time in practice. Building permit plan check and construction come after approval.
Do I need a permit to convert my garage into an ADU?
Yes. A garage with a kitchen and bathroom becomes a dwelling unit, and the City's code requires a building permit for any ADU or JADU, with plumbing, electrical and mechanical work inspected. Converting without one leaves you with a unit you cannot safely rent or sell. Ask Planning about parking and the garage's size rules.
Tell us about your El Cajon ADU plans
Tell us the neighborhood, whether you are weighing the garage or the back yard, and how old the house is and whether the lot is flat or on a slope, and we will plan the first visit around the City's review.