What happens if you skip the permit (and you needed one)
- Stop-work order issued by LA County Building & Safety can halt framing mid-cycle; re-permitting costs double the original permit fee ($8,000–$15,000 total) plus $1,000–$3,000 in re-inspection fines.
- Title company refuses to insure the property when selling; buyer's lender will not fund until ADU is permitted retroactively, adding 4-6 months and $10,000+ in remediation costs.
- Insurance claim denial if fire or structural damage occurs; your homeowner's policy explicitly excludes unpermitted structures.
- HOA and neighbor complaints trigger code enforcement complaint; City of San Dimas issues a Notice of Violation and forces removal of non-compliant unit within 30 days or faces liens up to $5,000.
San Dimas ADU permits — the key details
California Government Code 65852.2 (primary ADU law) and 65852.22 (junior ADU law) set the state floor: cities must allow one ADU per lot zoned for single-family use; second units are allowed if owner-occupied; junior ADUs (smaller, within existing structure) get even broader latitude. San Dimas cannot impose owner-occupancy on the primary residence in the same lot, though it can require the owner to occupy ONE unit (primary or ADU). The city's 2019 California Building Code incorporates these mandates. IRC R310.1 requires egress windows (5.7 sq ft openable, or 5% of floor area) in all bedrooms; IRC R404 mandates foundation design for detached structures (frost depth in San Dimas foothills reaches 12-30 inches at elevation). San Dimas plan review typically flagged detached ADUs for setback compliance — minimum 5 feet from side/rear property lines under local code, though state law allows reduced setbacks if supported by the local design guidelines. The key: state law preempts local setback rules if the local rule is stricter than state floors, so challenge any 10-foot setback demand from the city if your design meets the state-mandated 5 feet.
San Dimas Building Department operates a 60-day shot clock for ministerial ADU review (AB 671 compliance). This means if you submit a complete application with title report, survey, site plan showing setbacks, utility plans, and architectural drawings, the city has 60 calendar days to approve or deny — no discretionary delays, no extended plan review for general plan consistency. In practice, most San Dimas ADU applications finish in 70-90 days because the 'complete' threshold requires a Title Company report verifying owner occupancy intent and a certified survey or recent parcel map showing lot dimensions and existing structures. Detached ADUs require full structural engineer seismic design (California Building Code Chapter 12, seismic design category D2 for LA County foothills); this adds 2-3 weeks to plan preparation and $1,500–$3,000 in engineering fees. Garage conversions skip structural seismic redesign but require new egress windows (not a 'conversion-exempt' loophole in San Dimas) and electrical panel upgrade (adding 2-3 days to permitting once submitted). The city's online portal is accessed through the main San Dimas city website; many applicants in 2023-2024 report faster turnaround filing through the portal vs. in-person.
Utility connections are a common hold-up. San Dimas municipal water and sewer serve most of the town proper; foothills properties (unincorporated county areas within the San Dimas area) may rely on well and septic. If you're on septic, the ADU triggers San Dimas County Health Department (not City) review — septic system must be sized for the combined household (main + ADU), requiring a septic system engineer's report (add $2,000–$4,000 and 4 weeks). On-grid ADUs need separate utility meters or sub-meters; San Dimas Department of Water & Power (DWP) and the local gas company must issue 'will-serve' letters showing capacity before the city signs off on building permits. This alone can take 3-6 weeks in 2024 because DWP is processing high ADU volume in LA County. Junior ADUs (under 500 sq ft, within the existing primary residence) are exempt from separate utility requirements in California state law, so if you're considering that route, you skip this wait — but junior ADUs must share utilities with the primary unit, meaning no separate meter and lower rentability.
Parking is waived for owner-occupied ADUs in California state law (Gov. Code 65852.2(c)). San Dimas cannot require parking spaces for an ADU if the owner occupies either the primary residence or the ADU. If you're renting out both (non-owner-occupied), the city may impose 1 space per unit, but this is uncommon for single-lot ADUs; most applicants claim owner-occupancy and waive parking. Setback and lot-size rules: San Dimas local code requires ADUs on lots ≥6,000 sq ft for detached units and ≥5,000 sq ft for garage conversions. These are NOT per state law; state law sets no lot-size minimum for ADUs. However, San Dimas's 5-foot side/rear setback is roughly aligned with state defaults, so you're unlikely to hit a conflict here. Detached ADUs in the 600-1,200 sq ft range are most common in San Dimas foothills; above 1,200 sq ft, plan review time increases due to more complex structural and fire-life-safety review.
Owner-builder rules for ADUs: California B&P Code § 7044 allows homeowners to build ADUs on their own property without a contractor license, BUT trades (electrical, plumbing, HVAC) must be licensed. San Dimas enforces this strictly — expect the inspector to ask for licensed electrician affidavit and plumber sign-off at rough-in and final. Many owner-builders hire a 'GC shepherd' (unlicensed general contractor) who coordinates subs; this is gray-area legal in CA and common in San Dimas, but officially the city prefers a licensed GC. If you go owner-builder route, budget extra time (10-14 weeks) because inspections may be more rigorous and sub-trades must show licenses. Overall timeline for a detached ADU in San Dimas: pre-design and survey (4 weeks), design and permitting (8-12 weeks including city's 60-day clock), construction (16-20 weeks for 800 sq ft), and close-out (2 weeks). Total: 6-7 months from first sketch to certificate of occupancy.
Three San Dimas accessory dwelling unit (adu) scenarios
California state law override: why San Dimas cannot reject your ADU, even if it wanted to
In 2017, California passed SB 1069, which became Government Code 65852.2. This law stripped cities of the power to prohibit ADUs (also called 'second units' or 'in-law units') in single-family zoned areas. San Dimas, historically a no-ADU city in its 1998 general plan, was forced to update its municipal code to accept ADU applications by 2018. In 2019, AB 68 further tightened the rule: cities cannot impose 'unreasonable' design standards or parking on ADUs. Then in 2021, AB 881 added junior ADUs (small ADUs within existing structures) with even looser rules. The cumulative effect: if you own single-family zoned property in San Dimas, the city MUST process your ADU application under a ministerial 60-day clock (AB 671, effective 2022). Ministerial review means the city cannot deny your application based on discretionary criteria like 'general plan consistency' or 'neighborhood character' — only objective standards (setback distance, parking count, egress window size) matter.
San Dimas Building Department officially adopted these state laws into the 2019 California Building Code edition, which they use today. The critical clause in the city's ADU ordinance (codified in San Dimas Municipal Code or the Building Code adoption resolution) is usually something like: 'ADU applications shall be processed ministerially in compliance with Government Code 65852.2 and 65852.22; approval shall be granted if the application demonstrates compliance with setback, utility, and design standards listed in Appendix ADU.' This means the city CANNOT impose extra conditions (like a design review, traffic study, or CEQA analysis) that would delay you beyond the 60-day clock. If you encounter pushback from a junior city planner saying 'we need time to study neighborhood impact,' you can cite the state law and demand clock compliance.
One nuance: owner-occupancy. State law does NOT require the owner to live in the ADU itself, but it does require the owner to occupy ONE of the two units (primary residence or ADU). So you cannot build two detached ADUs on one lot and rent both out; at least one must be owner-occupied. Non-owner-occupied ADUs (both units rented) are allowed in limited cases under 65852.2(e), mainly in higher-opportunity areas or if the property is zoned for multi-family. San Dimas has not widely opened the non-owner-occupied door, so assume your ADU must be owner-occupied (or owner of the property must live in the primary residence). This restriction does NOT apply to junior ADUs (state law allows junior ADUs to be rented out if the primary residence is owner-occupied). Scenario C above is non-owner-occupied because I interpreted 'owner-occupied' as the owner living there, but practically, many San Dimas applicants claim owner-occupancy for the primary residence and then rent the ADU; verify with the city if you're in a gray area.
Seismic design, septic systems, and the foothills factor: what changes when your ADU is 1,500 feet up
San Dimas spans about 600-2,800 feet elevation. Central San Dimas (near the main commercial corridor and downtown) is ~750 ft and sits in seismic design category D2 per USGS. The San Dimas foothills (north and northeast, toward the San Gabriel Mountains) reach 1,500-2,800 ft and are still D2, but with steeper grades, granitic soil (good bearing capacity), and frost depth 12-30 inches — far different from coastal Los Angeles, which has 0-4 inch frost due to marine influence. For detached ADUs in the foothills (Scenarios A and C), this means you need geotechnical engineer review if the lot has >10% grade. California Building Code Chapter 18 (soils and foundations) requires a soils report for structures on slopes; cost is $1,500–$3,000 for a engineer's site investigation + lab testing. Slab-on-grade ADUs on steep foothills lots often require post & pier or grade-beam foundations to avoid differential settling. The frost depth (12-30 inches at higher elevations) does NOT apply to slab-on-grade (which floats), but does apply to post & pier or trench foundation for utilities; footings must be below frost line. San Dimas foothills contractors know this, but plan-review delays happen when applicants submit slab designs for steep lots without a geotechnical report.
Septic systems: much of San Dimas foothills is unincorporated LA County without municipal sewer. If your property is on septic (as in Scenario A), the ADU triggers San Dimas County Health Department (not City Building) review. Existing septic tank + drain field are typically sized for a 3-bedroom, 2-bath primary home. A 2-bedroom ADU adds load equivalent to ~1 bedroom (100 gallons per day per bedroom as rule-of-thumb in California code). So a 1,000-gallon tank serving a 3-bed house (300 gpd) is at 75% capacity and may not have room for a 2-bed ADU (200 gpd additional, pushing total to 500 gpd = 50% capacity on a 1,000 gal tank, which is acceptable). But if the existing tank is 750 gal (older homes), you likely need an additional 500-750 gal tank or replacement of the whole system (cost $10,000–$20,000). County Health requires a septic system engineer to certify this; do NOT skip this step. Some foothills properties have private (non-county) septic contracts; verify your county Health jurisdiction before design begins.
Well water: many foothills properties pump from private wells. ADUs on-well are allowed in California, but the well must serve both structures. If your existing well has low yield (common in granitic soils — sometimes <5 gpm), adding an ADU may require a new well. LA County Department of Public Works issues well permits; typical cost $3,000–$8,000 to drill a new well and install a UV or sand filter. Minimum well yield for an ADU is ~5-10 gpm in most county interpretations; your existing well should be tested before you commit to an ADU design. Finally, foothills ADUs may trigger fire district review if the property is in a State Responsibility Area (SRA) or Local Responsibility Area (LRA) fire zone. San Dimas foothills are predominantly SRA; CAL FIRE (California Department of Forestry) may impose defensible space, roof materials, or fence setback requirements for new structures. This is separate from the building permit but can delay site design if not caught early. Bottom line: foothills ADUs require 12-16 weeks permitting vs. 8-10 weeks in central San Dimas.
245 E. Foothill Boulevard, San Dimas, CA 91773
Phone: (909) 394-6200 | https://www.ci.san-dimas.ca.us/departments/public-works-and-engineering/building-and-safety
Monday – Friday, 8:00 AM – 5:00 PM (closed holidays; verify hours on city website)
Common questions
Can I build an ADU on my San Dimas property if the lot is only 0.3 acres?
San Dimas local code sets a minimum 6,000 sq ft (≈0.138 acres) for detached ADUs and 5,000 sq ft for garage conversions. A 0.3-acre lot (≈13,000 sq ft) meets both thresholds. However, if your lot is ≤5,000 sq ft, you're limited to a junior ADU (within the existing primary residence), which is allowed by state law even on very small lots. The City cannot deny a junior ADU on a lot smaller than 5,000 sq ft — state law preempts local lot-size rules for junior ADUs.
Do I have to live in the ADU myself, or can I rent it out immediately?
California state law (Government Code 65852.2) requires that the property owner occupy either the primary residence or the ADU; you cannot rent both out on a single lot. However, many San Dimas homeowners achieve this by living in the primary residence and renting the ADU long-term. If the property is non-owner-occupied (you own it but live elsewhere), you generally cannot add an ADU in San Dimas under current state law, with rare exceptions in high-opportunity census tracts. Junior ADUs have looser rules: if the primary residence is owner-occupied, the junior ADU can be rented out. Confirm with the City of San Dimas Building Department at (909) 394-6200 before you submit, as interpretation of owner-occupancy requirements can vary.
How long does a San Dimas ADU permit take, and what's the process?
San Dimas operates a 60-calendar-day ministerial review clock under Assembly Bill 671. Once you submit a complete application (title report, survey, site plan, architectural drawings, utility plans, structural engineer seismic design if detached), the city has 60 days to issue or deny the permit. In practice, most applications take 70-90 days because the city may need 2-3 weeks to deem your application 'complete.' Detached ADUs on foothills properties with septic systems can stretch to 12-14 weeks if county Health review adds time. After permit issuance, construction typically takes 14-20 weeks, followed by final inspections and certificate of occupancy. Total project: 6-7 months.
Do I need a separate electrical meter and water meter for my ADU?
Detached ADUs and garage conversions require separate water and sewer service lines in most California cities, but NOT separate meters (unless your utility company mandates one for billing). San Dimas Department of Water & Power and local gas company allow sub-metering or a single meter serving both structures, though separate meters are common for rental clarity. Junior ADUs are exempt from separate utility lines under state law — a junior ADU must share utilities with the primary residence and cannot have a separate meter. Check with DWP before design if you want separate metering; they typically charge $2,000–$4,000 to run a new service line and meter.
What if my San Dimas property is in an unincorporated area or on septic?
If your property is in unincorporated LA County (not within city limits), you apply for building permits through LA County Building and Safety, not the City of San Dimas. County ADU rules align with state law but may have different fee schedules and timelines. If your property is on septic (common in foothills), San Dimas County Health Department reviews the septic system design separately from the building permit — allow an extra 4-6 weeks for Health approval. You need a septic system engineer to certify that the existing system (or a new/expanded system) can serve the combined household. Do not assume your existing septic tank is adequate; it often is not.
Is parking required for my ADU in San Dimas?
No parking is required for owner-occupied ADUs under California state law (Government Code 65852.2(c)). San Dimas cannot require parking spaces for a single ADU on a lot if the owner occupies either the primary residence or the ADU. If your ADU is non-owner-occupied (rare scenario), the city may impose 1 space per unit, but this is unusual on single-lot properties. Parking waivers are effectively automatic for owner-occupied ADUs in San Dimas.
What is a 'ministerial' review, and why does it matter for my ADU?
Ministerial review means the city must approve your ADU application if it complies with objective standards (setback distance, window size, utility access) — the city cannot deny it based on subjective criteria like 'neighborhood character' or 'general plan consistency.' California law requires a 60-day ministerial review for ADUs. This protects you from long delays while the city 'studies' your project. If San Dimas tries to impose a design review, traffic analysis, or CEQA review on your ADU, you can cite AB 671 and demand ministerial clock compliance.
Can I act as my own general contractor (owner-builder) for a San Dimas ADU?
Yes. California B&P Code § 7044 allows homeowners to build ADUs without a contractor license. However, all licensed trades (electrical, plumbing, HVAC, gas) must be performed by licensed contractors. San Dimas Building Department will require evidence of licensed sub-trade work (affidavits, licenses) at rough-in and final inspections. Owner-builder projects may take 1-2 weeks longer because inspectors often scrutinize workmanship more closely. Many owner-builders hire a 'GC coordinator' (unlicensed) to manage subs; this is common practice but technically gray-area under state law.
What are egress windows, and why does my ADU need them?
Egress windows (or egress doors) are required emergency exits in bedrooms per IRC R310.1. Each bedroom must have an operable window ≥5.7 sq ft opening area (roughly 2 ft wide × 3 ft tall) OR a door to the outside. For ADUs in garages or above-garage structures, you often need to cut a new window opening, which adds cost ($1,500–$2,500) and time (1-2 weeks). Some ADU designs use a second door (to a deck or stairwell) to avoid new window cutting. Verify egress options early in design to avoid expensive re-planning.
What are the biggest reasons San Dimas denies or delays ADU permits?
Incomplete applications (missing survey, title report, or utility will-serve letters) are the #1 cause of delays. Setback violations on small lots or constrained properties (detached ADUs too close to property lines) trigger denials. Septic system inadequacy or lack of engineer certification delays foothills projects by 4-8 weeks. Utility company delays (DWP taking 6+ weeks to issue will-serve) are outside the city's control but common in 2024. Finally, applicants claiming owner-occupancy without adequate documentation (deed, occupancy intent letter) can face scrutiny. Hire a plan-prep specialist or expeditor if your property is complex; $1,000–$2,000 upfront saves weeks of back-and-forth with the city.