San Dimas Garage Conversion ADU 2026 | $120K-$260K, San Dimas Permits

A San Dimas garage conversion ADU has two extra constraints beyond the standard ADU build. First, if the garage sits on a lot inside the VHFHSZ (which much of upper 91773 does), the conversion has to meet Chapter 7A fire-hardening standards for the dwelling unit — Class A roof, ignition-resistant cladding, ember-resistant venting. Second, the foothill build is slower in winter and the lot conditions (slope, septic, longer driveway pulls for material delivery) add coordination that a flat-lot ADU does not face. With those two acknowledged, the garage conversion still makes sense in San Dimas — multi-generational housing for grandparents moving from elsewhere in LA, an income unit for households planning to move up further into the foothills, or a JADU/ADU income split. NPLD has been designing in Los Angeles since 2016 and licensed as a CSLB General Contractor since 2023, with over 200 LA County builds completed. Our San Dimas garage conversion ADUs run $120K-$260K over a 10-18 week build, pulled through the City of San Dimas Building Division at 245 E. Bonita Ave.

What a San Dimas Garage Conversion ADU Costs in 2026

Three honest tiers. The entry tier, $120K-$160K, is a clean two-car garage conversion to a 400-450 sf studio: insulation envelope, new windows and a residential entry door where the garage door was, compact kitchen, full bath with curbless walk-in shower, dedicated sub-panel, mini-split HVAC. If the lot is VHFHSZ-adjacent, Chapter 7A adds $8K-$18K for ignition-resistant cladding and vent retrofits. The mid tier, $160K-$210K, is a 500-600 sf one-bedroom with a real kitchen, separate bedroom, larger bath, in-unit washer-dryer. The top tier, $210K-$260K, is a 600-700 sf premium build with custom cabinetry, quartz counters, a small private outdoor patio, and integrated landscape. Permits, school fees, and Title 24 2022 documentation add $5K-$12K. State ADU fee waivers apply for units under 750 sf. If the property is on septic, septic capacity review and potential upgrade adds $4K-$22K.

Chapter 7A on the Garage Conversion — What It Actually Means

If the existing garage is inside the VHFHSZ, converting it to a dwelling unit triggers Chapter 7A. The garage was probably built before Chapter 7A was on the books, so the existing roof, eaves, walls, and vents likely do not meet current standards. The conversion has to bring the dwelling-unit envelope up to code. The roof either needs to be Class A non-combustible already (asphalt shingle Class A, tile, metal) or it needs to be re-roofed as part of the conversion. The eaves need to be ignition-resistant — usually fully boxed soffit with non-combustible material. Wall cladding on the dwelling envelope needs to be ignition-resistant (stucco qualifies, fiber-cement qualifies). The new windows replacing the garage door need to be tempered dual-pane minimum, sometimes laminated. The attic and crawlspace vents need ember-resistant mesh. This is real work and it is non-negotiable on a VHFHSZ lot. We walk the household through what the existing structure has and what needs to change at design intake. If your lot is not in the VHFHSZ — large parts of lower-elevation 91773 are outside it — Chapter 7A does not apply and the build runs closer to the East SGV baseline.

Septic, Foothill Lot Conditions, and Why the Build Takes a Month Longer

Many San Dimas lots in the upper foothill areas are on septic rather than city sewer. Adding a dwelling unit (with its own bath and kitchen) adds load to the septic system. We coordinate with a licensed septic engineer at design intake to confirm capacity. If the existing septic is sized for a 4-person main house and the ADU adds 2 more, the tank or leach field may need an upgrade — $4K-$22K depending on system age and condition. The foothill lot also affects material delivery. A long driveway pull means the demolition dumpster, the new windows, the kitchen cabinets, and the HVAC condenser all have to make it to the garage without damaging the household's landscape or the neighbor's. We use smaller delivery trucks and stage materials on the household's schedule. The foothill winter weather can shut down exterior work for 1-2 weeks during major storms — we schedule demo and rough-MEP for the dry-weather windows so the interior progress does not stall.

• Chapter 7A roof retrofit (if existing roof is not Class A): $14-$28 per sf of roof area
• Chapter 7A wall cladding upgrade (stucco or fiber-cement): $9-$18 per sf of wall area
• Ember-resistant attic and crawlspace vent retrofit: $1.8K-$4.5K
• Septic engineer review and potential upgrade: $4K-$22K depending on system
• Tempered dual-pane Chapter 7A windows: $900-$2,200 per window
• Mini-split HVAC: $5K-$9K

San Dimas Permits and Build Sequencing

San Dimas Building Division ADU permits run through a state-mandated streamlined review under SB 9, AB 68, and related statutes — plan check capped at 60 days by law. In practice, complete packages clear in 5-8 weeks (the foothill jurisdictions are sometimes slightly slower than flat-lot cities). On VHFHSZ lots, the package also coordinates with LA County Fire for Chapter 7A sign-off. We sequence the permit packet to go in finished: structural sheet, electrical load calc, Title 24 2022 compliance, plumbing diagram, septic engineer's letter where applicable, and Chapter 7A specification sheet. Once permits clear, construction runs 10-18 weeks. The main house is unaffected — all demo, framing, and finish happen inside the garage envelope. The same foreman runs the job from intake to final inspection. Final inspection clears the dwelling unit for occupancy and (where applicable) for separate-lot title under AB1033.